Allow albums to reuse another album's wallpaper

[towerofnix]

I don't really feel like restructuring the media folder yet - better to keep this in mind for #70.

But as seen in hsmusic/hsmusic-data#211, album wallpapers/backgrounds can end up being quite heavyweight files, occasionally unavoidably (it's often displayed at massive size, so is more sensitive to low-resolution JPEG artifact upscaling).

In cases where two albums share a wallpaper, those have to be saved (and requested!) as separate assets, currently. There should be a field for snagging the asset from another album's media folder, so it only needs to be requested once.

Prolly Reuse Wallpaper Asset: Psycholonials EP 1 or such.

[towerofnix]

We're closing this in favor of #545!