MM-40818: Refactor product initialization to happen late (#19658)

This is more of a general refactor of the initialization
process which should allow us to pass services more
easily.

The changes are minimal to keep the scope limited.
For now, the objective is to pass the file service
to the Channels product. For that, it was required
to move some of the enterprise interfaces under Channels
from Server.

We also create a filestore field in the server to
avoid creating filestore reference every time
we make a filestore operation. This will be later
passed on to the Channels product.

Also removed an unnecessary test. 
The test was working so far because we were creating
the filebackend every time for every request. But
we should go via UpdateConfig call which would fail,
were we to assign an invalid filestore name.

So we were actually testing for a different thing.
Therefore, removed the test.

```release-note
NONE
```

app/file.go

@@ -47,7 +47,7 @@ const (
 	maxContentExtractionSize   = 1024 * 1024 // 1MB
 )
 
-func (a *App) FileBackend() (filestore.FileBackend, *model.AppError) {
+func (a *App) FileBackend() filestore.FileBackend {
 	return a.Srv().FileBackend()
 }
 
@@ -72,11 +72,7 @@ func connectionTestErrorToAppError(connTestErr error) *model.AppError {
 }
 
 func (a *App) TestFileStoreConnection() *model.AppError {
-	backend, err := a.FileBackend()
-	if err != nil {
-		return err
-	}
-	nErr := backend.TestConnection()
+	nErr := a.FileBackend().TestConnection()
 	if nErr != nil {
 		return connectionTestErrorToAppError(nErr)
 	}
@@ -101,11 +97,7 @@ func (a *App) ReadFile(path string) ([]byte, *model.AppError) {
 }
 
 func (s *Server) fileReader(path string) (filestore.ReadCloseSeeker, *model.AppError) {
-	backend, err := s.FileBackend()
-	if err != nil {
-		return nil, err
-	}
-	result, nErr := backend.Reader(path)
+	result, nErr := s.FileBackend().Reader(path)
 	if nErr != nil {
 		return nil, model.NewAppError("FileReader", "api.file.file_reader.app_error", nil, nErr.Error(), http.StatusInternalServerError)
 	}
@@ -122,11 +114,7 @@ func (a *App) FileExists(path string) (bool, *model.AppError) {
 }
 
 func (s *Server) fileExists(path string) (bool, *model.AppError) {
-	backend, err := s.FileBackend()
-	if err != nil {
-		return false, err
-	}
-	result, nErr := backend.FileExists(path)
+	result, nErr := s.FileBackend().FileExists(path)
 	if nErr != nil {
 		return false, model.NewAppError("FileExists", "api.file.file_exists.app_error", nil, nErr.Error(), http.StatusInternalServerError)
 	}
@@ -134,11 +122,7 @@ func (s *Server) fileExists(path string) (bool, *model.AppError) {
 }
 
 func (a *App) FileSize(path string) (int64, *model.AppError) {
-	backend, err := a.FileBackend()
-	if err != nil {
-		return 0, err
-	}
-	size, nErr := backend.FileSize(path)
+	size, nErr := a.FileBackend().FileSize(path)
 	if nErr != nil {
 		return 0, model.NewAppError("FileSize", "api.file.file_size.app_error", nil, nErr.Error(), http.StatusInternalServerError)
 	}
@@ -146,11 +130,7 @@ func (a *App) FileSize(path string) (int64, *model.AppError) {
 }
 
 func (a *App) FileModTime(path string) (time.Time, *model.AppError) {
-	backend, err := a.FileBackend()
-	if err != nil {
-		return time.Time{}, err
-	}
-	modTime, nErr := backend.FileModTime(path)
+	modTime, nErr := a.FileBackend().FileModTime(path)
 	if nErr != nil {
 		return time.Time{}, model.NewAppError("FileModTime", "api.file.file_mod_time.app_error", nil, nErr.Error(), http.StatusInternalServerError)
 	}
@@ -159,11 +139,7 @@ func (a *App) FileModTime(path string) (time.Time, *model.AppError) {
 }
 
 func (a *App) MoveFile(oldPath, newPath string) *model.AppError {
-	backend, err := a.FileBackend()
-	if err != nil {
-		return err
-	}
-	nErr := backend.MoveFile(oldPath, newPath)
+	nErr := a.FileBackend().MoveFile(oldPath, newPath)
 	if nErr != nil {
 		return model.NewAppError("MoveFile", "api.file.move_file.app_error", nil, nErr.Error(), http.StatusInternalServerError)
 	}
@@ -175,12 +151,7 @@ func (a *App) WriteFile(fr io.Reader, path string) (int64, *model.AppError) {
 }
 
 func (s *Server) writeFile(fr io.Reader, path string) (int64, *model.AppError) {
-	backend, err := s.FileBackend()
-	if err != nil {
-		return 0, err
-	}
-
-	result, nErr := backend.WriteFile(fr, path)
+	result, nErr := s.FileBackend().WriteFile(fr, path)
 	if nErr != nil {
 		return result, model.NewAppError("WriteFile", "api.file.write_file.app_error", nil, nErr.Error(), http.StatusInternalServerError)
 	}
@@ -188,12 +159,7 @@ func (s *Server) writeFile(fr io.Reader, path string) (int64, *model.AppError) {
 }
 
 func (a *App) AppendFile(fr io.Reader, path string) (int64, *model.AppError) {
-	backend, err := a.FileBackend()
-	if err != nil {
-		return 0, err
-	}
-
-	result, nErr := backend.AppendFile(fr, path)
+	result, nErr := a.FileBackend().AppendFile(fr, path)
 	if nErr != nil {
 		return result, model.NewAppError("AppendFile", "api.file.append_file.app_error", nil, nErr.Error(), http.StatusInternalServerError)
 	}
@@ -205,11 +171,7 @@ func (a *App) RemoveFile(path string) *model.AppError {
 }
 
 func (s *Server) removeFile(path string) *model.AppError {
-	backend, err := s.FileBackend()
-	if err != nil {
-		return err
-	}
-	nErr := backend.RemoveFile(path)
+	nErr := s.FileBackend().RemoveFile(path)
 	if nErr != nil {
 		return model.NewAppError("RemoveFile", "api.file.remove_file.app_error", nil, nErr.Error(), http.StatusInternalServerError)
 	}
@@ -221,11 +183,7 @@ func (a *App) ListDirectory(path string) ([]string, *model.AppError) {
 }
 
 func (s *Server) listDirectory(path string) ([]string, *model.AppError) {
-	backend, err := s.FileBackend()
-	if err != nil {
-		return nil, err
-	}
-	paths, nErr := backend.ListDirectory(path)
+	paths, nErr := s.FileBackend().ListDirectory(path)
 	if nErr != nil {
 		return nil, model.NewAppError("ListDirectory", "api.file.list_directory.app_error", nil, nErr.Error(), http.StatusInternalServerError)
 	}
@@ -234,11 +192,7 @@ func (s *Server) listDirectory(path string) ([]string, *model.AppError) {
 }
 
 func (a *App) RemoveDirectory(path string) *model.AppError {
-	backend, err := a.FileBackend()
-	if err != nil {
-		return err
-	}
-	nErr := backend.RemoveDirectory(path)
+	nErr := a.FileBackend().RemoveDirectory(path)
 	if nErr != nil {
 		return model.NewAppError("RemoveDirectory", "api.file.remove_directory.app_error", nil, nErr.Error(), http.StatusInternalServerError)
 	}