Keep required users

This should prevent the last author and copyright holder from being removed (from the UI).  I'm having some trouble getting the stub accounts in cnxauthoring to work, so I wasn't able to fully test this (adding a user should allow both users to be removed and should only lock when only one user remains).

There should probably also be a hard check on the backend.

Branches off #733 

https://trello.com/c/eTib1Uag/877-add-and-remove-users-in-different-attributions-on-modules-the-attributions-include-editor-author-translator-etc-8-pts