WIP: Allow users to upload media to shared albums

[the01]

Basic working implementation for #911

@derneuere could you please take a look? As a first step, I took the simplest approach to get this feature working without changing the FE, so probably a few adjustments are needed.

I added a SHARED_ALBUM_ALL_ADD setting with an env variable of the same name, so that admins can decide if the basic functionality works for them or they want to wait until the edge cases are also covered.

Do we need to filter here again even though we already did it here?