text: Title: Notify the user to activate MANAGE_EXTERNAL_STORAGE
  Body: **Describe the enhancement you'd like**
  It looks like you can only activate MANAGE_EXTERNAL_STORAGE in the settings. Notify the user to do that, if he wants to delete backed up images.
  
  **Describe why this will benefit the LibrePhotos**
  This will lead to less confusion on why the app behaves the way it does.
  
  **Additional context**
  Follow-up issue from #736 