Move pluginCommands into Channels (#18974)

* Move pluginCommands into Channels

We move pluginCommands, pluginCommandsLock
into Channels.

We also move the plugin related route handlers
under Channels and move the init code under
NewChannels. To achieve this, the router initialization
is bumped up.

Along with it, we clean up some App methods
which were just wrappers over Channel methods.
Instead, we call the Channel method directly
to make things more readable and easy to understand.

```release-note
NONE
```

* fix tests

```release-note
NONE
```

app/server.go

@@ -142,9 +142,6 @@ type Server struct {
 	configStore             *config.Store
 	postActionCookieSecret  []byte
 
-	pluginCommands     []*PluginCommand
-	pluginCommandsLock sync.RWMutex
-
 	telemetryService *telemetry.TelemetryService
 	userService      *users.UserService
 	teamService      *teams.TeamService
@@ -213,6 +210,7 @@ func NewServer(options ...Option) (*Server, error) {
 		licenseListeners:    map[string]func(*model.License, *model.License){},
 		hashSeed:            maphash.MakeSeed(),
 		uploadLockMap:       map[string]bool{},
+		timezones:           timezones.New(),
 		products:            make(map[string]Product),
 	}
 
@@ -245,6 +243,12 @@ func NewServer(options ...Option) (*Server, error) {
 		mlog.Error("Could not initiate logging", mlog.Err(err))
 	}
 
+	subpath, err := utils.GetSubpathFromConfig(s.Config())
+	if err != nil {
+		return nil, errors.Wrap(err, "failed to parse SiteURL subpath")
+	}
+	s.Router = s.RootRouter.PathPrefix(subpath).Subrouter()
+
 	// This is called after initLogging() to avoid a race condition.
 	mlog.Info("Server is initializing...", mlog.String("go_version", runtime.Version()))
 
@@ -253,9 +257,9 @@ func NewServer(options ...Option) (*Server, error) {
 	// Step 3: Initialize products.
 	// Depends on s.httpService.
 	for name, initializer := range products {
-		prod, err := initializer(s)
-		if err != nil {
-			return nil, errors.Wrapf(err, "error initializing product: %s", name)
+		prod, err2 := initializer(s)
+		if err2 != nil {
+			return nil, errors.Wrapf(err2, "error initializing product: %s", name)
 		}
 
 		s.products[name] = prod
@@ -280,8 +284,8 @@ func NewServer(options ...Option) (*Server, error) {
 	// At the moment we only have this implementation
 	// in the future the cache provider will be built based on the loaded config
 	s.CacheProvider = cache.NewProvider()
-	if err := s.CacheProvider.Connect(); err != nil {
-		return nil, errors.Wrapf(err, "Unable to connect to cache provider")
+	if err2 := s.CacheProvider.Connect(); err2 != nil {
+		return nil, errors.Wrapf(err2, "Unable to connect to cache provider")
 	}
 
 	// It is important to initialize the hub only after the global logger is set
@@ -326,7 +330,6 @@ func NewServer(options ...Option) (*Server, error) {
 		}
 	}
 
-	var err error
 	s.Store, err = s.newStore()
 	if err != nil {
 		return nil, errors.Wrap(err, "cannot create store")
@@ -532,17 +535,6 @@ func NewServer(options ...Option) (*Server, error) {
 		return nil, errors.Wrapf(err, "unable to ensure first run timestamp")
 	}
 
-	subpath, err := utils.GetSubpathFromConfig(s.Config())
-	if err != nil {
-		return nil, errors.Wrap(err, "failed to parse SiteURL subpath")
-	}
-	s.Router = s.RootRouter.PathPrefix(subpath).Subrouter()
-
-	pluginsRoute := s.Router.PathPrefix("/plugins/{plugin_id:[A-Za-z0-9\\_\\-\\.]+}").Subrouter()
-	pluginsRoute.HandleFunc("", s.ServePluginRequest)
-	pluginsRoute.HandleFunc("/public/{public_file:.*}", s.ServePluginPublicRequest)
-	pluginsRoute.HandleFunc("/{anything:.*}", s.ServePluginRequest)
-
 	// If configured with a subpath, redirect 404s at the root back into the subpath.
 	if subpath != "/" {
 		s.RootRouter.NotFoundHandler = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
@@ -580,7 +572,6 @@ func NewServer(options ...Option) (*Server, error) {
 		}
 	}
 
-	s.timezones = timezones.New()
 	// Start email batching because it's not like the other jobs
 	s.AddConfigListener(func(_, _ *model.Config) {
 		s.EmailService.InitEmailBatching()