Update content models to 27.2.0
Disable legacy 'nil' user account.

This account has no uid, name or email address and appears to be used
for unassigning content. It's no longer needed but it will have
  associations in other models so disable it.
Allow Editions to be unassigned.

Editions could formerly be assigned to a dummy user account as a
workaround to unassigning them.

- Update select2 assignee form control to allow easy unassignment.
- Update controller and workflow to allow real unassignment.
Merge pull request #333 from alphagov/unassign_user

Unassign an item in Publisher