Revert validation of album artists separator

[robbevp]

This reverts a small part of #722, where I moved the separator to the album artist model. This worked inside the model tests, but does not work in the controller since when updating (since we fully replace the album artists, during the validation album.album_artists.size is 0.

There are still two underlying issues that are not solved here:

• If we provide an invalid album artist, rails will raise an ActiveRecord::RecordNotSaved error and not render a body in the form that we expect. (I suspect this is the case for all associations that get replaced in our controllers: album labels, track artists, ...)
• We do properly handle this for create, but in that case the errors only contain { model: 'album', attribute: 'album_artists', type: 'invalid' }. So we have no context about which of the album artists is invalid and just what the issue is.
  I suspect that both of these issues where already present before Return errors as json objects instead of strings #722 - they are a bit trickier to solve, so I already wanted to get this quick fix out

• I've added tests relevant to my changes.