Remove the remaining fields from *app.Server (#19113)

We move these fields to Channels:
```
uploadLockMapMut sync.Mutex
uploadLockMap    map[string]bool
imgDecoder *imaging.Decoder
imgEncoder *imaging.Encoder
dndTaskMut sync.Mutex
dndTask    *model.ScheduledTask
```

I think this PR should conclue the initial phase
of migrating stuff from Server to Channels.

The remaining task would be to focus on continue
to create the remaining services from the
common things like users, teams, push notifications,
clustering for other products to consume.

https://community-daily.mattermost.com/boards/workspace/zyoahc9uapdn3xdptac6jb69ic/285b80a3-257d-41f6-8cf4-ed80ca9d92e5/495cdb4d-c13a-4992-8eb9-80cfee2819a4/87df1e15-588e-49ff-8bd1-ffa9651b8c82

```release-note
NONE
```

app/file.go

@@ -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.ch.srv.imgDecoder,
-		imgEncoder:  a.ch.srv.imgEncoder,
+		imgDecoder:  a.ch.imgDecoder,
+		imgEncoder:  a.ch.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.ch.srv.imgDecoder, bytes.NewReader(fileData[i]))
+		img, release, err := prepareImage(a.ch.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.ch.srv.imgEncoder.EncodeJPEG(&buf, imaging.GenerateThumbnail(img, imageThumbnailWidth, imageThumbnailHeight), jpegEncQuality); err != nil {
+	if err := a.ch.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.ch.srv.imgEncoder.EncodeJPEG(&buf, preview, jpegEncQuality); err != nil {
+	if err := a.ch.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.ch.srv.imgDecoder, file)
+		img, release, err := prepareImage(a.ch.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),