[Request] Make galleries able to belong to multiple categories

[HASJ]

Make an item be able to belong to multiple categories and be able to move physically move items on a category to a folder within its current place, if the user chooses to.
Example: "Western Cosplay Collection" belongs to the Cosplay and the Western Cosplay categories. Inside the Cosplay category you also selected Western Cosplay Collection along with a bunch of items in the Cosplay category, then show a prompt to select which folder Western Cosplay Collection will be physically moved to, Cosplay or Western Cosplay.

[twiddli]

Linking this, just in case #35