iOS: Improve user feedback that a photo has been queued for upload

When a photo is added to backup, there is no user feedback that anything happened. This applies in the single photo view as well as in the gallery view if offline.

Suggestions:

• In the single photo view, show a toast message as feedback that the photo has been added to the upload queue. Do not show the "Add to backup" menu item in the "3 dot" menu anymore.

• In the gallery view, show a different icon than "crossed out cloud" if the photo has been queued for upload but is not yet uploaded ("dashed cloud" maybe?).