MBS-6916: beta: No dropdown in inline search when there are no results

Adding the action buttons in _renderMenu was problematic because that method
isn't even called if there are no results (so it never gets a chance to add
the "(No Results)" button). So the buttons are now added in _lookupSuccess.

I also made a small adjustment to _searchTimeout so that the menu is properly
closed when the search input is cleared.
MBS-6918: artist credit changes break /release/add on beta

Problem 1:

I placed a copy of submitted_release_artist in the stash, but I added the
relevant line to prepare_tracklist in *ReleaseEditor::Edit*, which obviously
won't affect ReleaseEditor::Add. Now the line is in the main prepare_tracklist
function. (I haven't found out how this was working locally before, because I
specifically tested /release/add, but I must've changed something at the last
minute.)

Problem 2:

MB.Control.ArtistCredit expected initialData to not be an empty list. I'm not
sure if it's possible to be now that I've fixed problem 1, but just to be on
the safe side, I've modified Form::Field::ArtistCredit and
MB.Control.ArtistCredit to not return an empty list and not assume it's
dealing with a non-empty list, respectively.

Problem 3:

This escaped my initial testing. If you parse a tracklist with no artists,
the tracklist would no longer default to the release artist. That's now fixed.
Merge remote-tracking branch 'bitmap/mbs-6918' into beta

* bitmap/mbs-6918:
  MBS-6918: artist credit changes break /release/add on beta
Merge remote-tracking branch 'bitmap/mbs-6916' into beta

* bitmap/mbs-6916:
  MBS-6916: beta: No dropdown in inline search when there are no results