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/upload.go

@@ -166,23 +166,23 @@ func (a *App) GetUploadSessionsForUser(userID string) ([]*model.UploadSession, *
 func (a *App) UploadData(c *request.Context, us *model.UploadSession, rd io.Reader) (*model.FileInfo, *model.AppError) {
 	// prevent more than one caller to upload data at the same time for a given upload session.
 	// This is to avoid possible inconsistencies.
-	a.Srv().uploadLockMapMut.Lock()
-	locked := a.Srv().uploadLockMap[us.Id]
+	a.ch.uploadLockMapMut.Lock()
+	locked := a.ch.uploadLockMap[us.Id]
 	if locked {
 		// session lock is already taken, return error.
-		a.Srv().uploadLockMapMut.Unlock()
+		a.ch.uploadLockMapMut.Unlock()
 		return nil, model.NewAppError("UploadData", "app.upload.upload_data.concurrent.app_error",
 			nil, "", http.StatusBadRequest)
 	}
 	// grab the session lock.
-	a.Srv().uploadLockMap[us.Id] = true
-	a.Srv().uploadLockMapMut.Unlock()
+	a.ch.uploadLockMap[us.Id] = true
+	a.ch.uploadLockMapMut.Unlock()
 
 	// reset the session lock on exit.
 	defer func() {
-		a.Srv().uploadLockMapMut.Lock()
-		delete(a.Srv().uploadLockMap, us.Id)
-		a.Srv().uploadLockMapMut.Unlock()
+		a.ch.uploadLockMapMut.Lock()
+		delete(a.ch.uploadLockMap, us.Id)
+		a.ch.uploadLockMapMut.Unlock()
 	}()
 
 	// fetch the session from store to check for inconsistencies.