Move Channels into App (#18623)

* Move Channels into App

In this PR, we make Channels as part of App
instead of Server. This is part of the transition period
of moving fields from Server to Channels.

For now, Channels contains Server. So the hierarchy is

App -> Channels -> Server.

And as a first step, we also move httpService to Channels.

```release-note
NONE
```

* Fixing another test

```release-note
NONE
```

* new method

```release-note
NONE
```

app/file.go

@@ -98,7 +98,7 @@ func (a *App) TestFileStoreConnectionWithConfig(cfg *model.FileSettings) *model.
 }
 
 func (a *App) ReadFile(path string) ([]byte, *model.AppError) {
-	return a.srv.ReadFile(path)
+	return a.ch.srv.ReadFile(path)
 }
 
 func (s *Server) fileReader(path string) (filestore.ReadCloseSeeker, *model.AppError) {
@@ -703,8 +703,8 @@ func (a *App) UploadFileX(c *request.Context, channelID, name string, input io.R
 		Input:       input,
 		maxFileSize: *a.Config().FileSettings.MaxFileSize,
 		maxImageRes: *a.Config().FileSettings.MaxImageResolution,
-		imgDecoder:  a.srv.imgDecoder,
-		imgEncoder:  a.srv.imgEncoder,
+		imgDecoder:  a.ch.srv.imgDecoder,
+		imgEncoder:  a.ch.srv.imgEncoder,
 	}
 	for _, o := range opts {
 		o(t)
@@ -1040,7 +1040,7 @@ func (a *App) HandleImages(previewPathList []string, thumbnailPathList []string,
 	wg := new(sync.WaitGroup)
 
 	for i := range fileData {
-		img, release, err := prepareImage(a.srv.imgDecoder, bytes.NewReader(fileData[i]))
+		img, release, err := prepareImage(a.ch.srv.imgDecoder, bytes.NewReader(fileData[i]))
 		if err != nil {
 			mlog.Debug("Failed to prepare image", mlog.Err(err))
 			continue
@@ -1088,7 +1088,7 @@ func prepareImage(imgDecoder *imaging.Decoder, imgData io.ReadSeeker) (img image
 
 func (a *App) generateThumbnailImage(img image.Image, thumbnailPath string) {
 	var buf bytes.Buffer
-	if err := a.srv.imgEncoder.EncodeJPEG(&buf, imaging.GenerateThumbnail(img, imageThumbnailWidth, imageThumbnailHeight), jpegEncQuality); err != nil {
+	if err := a.ch.srv.imgEncoder.EncodeJPEG(&buf, imaging.GenerateThumbnail(img, imageThumbnailWidth, imageThumbnailHeight), jpegEncQuality); err != nil {
 		mlog.Error("Unable to encode image as jpeg", mlog.String("path", thumbnailPath), mlog.Err(err))
 		return
 	}
@@ -1103,7 +1103,7 @@ func (a *App) generatePreviewImage(img image.Image, previewPath string) {
 	var buf bytes.Buffer
 	preview := imaging.GeneratePreview(img, imagePreviewWidth)
 
-	if err := a.srv.imgEncoder.EncodeJPEG(&buf, preview, jpegEncQuality); err != nil {
+	if err := a.ch.srv.imgEncoder.EncodeJPEG(&buf, preview, jpegEncQuality); err != nil {
 		mlog.Error("Unable to encode image as preview jpg", mlog.Err(err), mlog.String("path", previewPath))
 		return
 	}
@@ -1124,7 +1124,7 @@ func (a *App) generateMiniPreview(fi *model.FileInfo) {
 			return
 		}
 		defer file.Close()
-		img, release, err := prepareImage(a.srv.imgDecoder, file)
+		img, release, err := prepareImage(a.ch.srv.imgDecoder, file)
 		if err != nil {
 			mlog.Debug("generateMiniPreview: prepareImage failed", mlog.Err(err),
 				mlog.String("fileinfo_id", fi.Id), mlog.String("channel_id", fi.ChannelId),