Add support for album inside albums

[GitGudGlenn]

On for example apple, you can create an album called "animals" and inside that album you can create other albums like " cats" and "dogs" and then inside those albums you can make albums for dogs: "jack", "sam" and then put the pictures of each specific dog inside their specific SORTED album tree.

Unless i've been looking around with my eyes shut, nextcloud doesn't allow you to sort images like this. This is also usefull for example for a holiday album with inside albums for each seperate holiday you've been to.

[luckkmaxx]

yeah this is a must-have