Notify users not in the proof workflow

[#1]

Currently, you can use @ to notify a user who is a current reviewer/owner of the proof.

For people with edit rights to the proof, they should be able to use @ to notify any user with Proof, adding them to the proof if they are not already. The person adding the user should be able to specify the role, notification settings and stage when they tag someone - the default should be an observer of the current stage.