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/user.go

@@ -767,7 +767,7 @@ func (a *App) SetProfileImageFromMultiPartFile(userID string, file multipart.Fil
 
 func (a *App) AdjustImage(file io.Reader) (*bytes.Buffer, *model.AppError) {
 	// Decode image into Image object
-	img, _, err := a.ch.srv.imgDecoder.Decode(file)
+	img, _, err := a.ch.imgDecoder.Decode(file)
 	if err != nil {
 		return nil, model.NewAppError("SetProfileImage", "api.user.upload_profile_user.decode.app_error", nil, err.Error(), http.StatusBadRequest)
 	}
@@ -780,7 +780,7 @@ func (a *App) AdjustImage(file io.Reader) (*bytes.Buffer, *model.AppError) {
 	img = imaging.FillCenter(img, profileWidthAndHeight, profileWidthAndHeight)
 
 	buf := new(bytes.Buffer)
-	err = a.ch.srv.imgEncoder.EncodePNG(buf, img)
+	err = a.ch.imgEncoder.EncodePNG(buf, img)
 	if err != nil {
 		return nil, model.NewAppError("SetProfileImage", "api.user.upload_profile_user.encode.app_error", nil, err.Error(), http.StatusInternalServerError)
 	}