Don't require missing AC fields for existing entities

If a form is editing an existing entity and the AC field is entirely
missing (as opposed to existing but being empty), the field will *not*
be required. The form will see this is as no changes being made. This
behavior allows bots and browser scripts to function properly (i.e.
environments where AC fields aren't generated).
Remove unused file
Fix ArtistCredit.js unit tests
Add tests for Form::Field::ArtistCredit
Add missing AC fields test for Controller::Recording::Edit
Merge remote-tracking branch 'bitmap/ac-bot-fix' into beta

* bitmap/ac-bot-fix:
  Add missing AC fields test for Controller::Recording::Edit
  Add tests for Form::Field::ArtistCredit
  Fix ArtistCredit.js unit tests
  Remove unused file
  Don't require missing AC fields for existing entities