Thumbnail: Rework cleanup

This MR reworks the thumbnail cleanup by introducing a new table dedicated to store the path of deleted thumbnail.

The previous way was to rely on the assumption that for each thumbnail, there would be an associated .jpg file in the thumbnail folder. This is false when:

• The thumbnail isn't owned
• The thumbnail isn't a jpg file (for instance, an embedded PNG thumbnail would be stored as a .png file)

At best, we were trying to delete files that didn't exist, at worst, we were deleting files that shouldn't have been deleted.

This should help with #350 (closed) but is probably not fixing it entirely

This is also likely to be related to #356 (closed)