Albums: Update UpdatedAt when photos are added/removed to/from an album

[graciousgrey]

As a user I want to see albums with newly added photos on top when filtering for "Recently Updated"

At the moment, the UpdatedAt field of albums is only updated when the user makes changes in the album's edit dialog (https://docs.photoprism.app/user-guide/organize/albums/#edit-album-details). It would be great if the UpdatedAt field would be additionally updated when photos are added to or removed from an album.

• The UpdatedAt field of an album (type album) MUST be updated when new photos are added
• The UpdatedAt field of an album (type album) MUST be updated when photos are removed from the album
• Write Unit Tests
• Update Documentation