Disable automatic addition/removal from candidates

The current version does the following:

1. Assigned users are automatically removed from candidates, deleting their note
2. Unassigned users are automatically added as candidates

I believe this is bad behavior, because

1. Notes are helpful even after translators have been assigned (and swapping assigned translators around removes the notes when we most need them)
2. Signing up as candidate should always be an active action from the translators, never from the admin

Maybe check with @teal but I strongly want to remove the automatic behavior.

We could consider moving already assigned candidates to the back of the list or maybe graying them out a bit?