When a field isn't lockable, just freeze the lock status instead of removing any lock

Summary:
See downstream issue here: https://phabricator.wikimedia.org/T150992

In at least one case (project milestones) we have a locked, non-lockable field. This means "this is locked, and you can't change the fact that it is locked".

At least for now, preserve this behavior.

Test Plan: Created a new milestone of an existing project. This worked correctly with the patch.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D16895