Make some Differential comment actions (like "Accept" and "Reject") conflict with one another

Summary:
Ref T11114. When a user selects "Accept", and then selects "Reject", remove the "Accept". It does not make sense to both accept and reject a revision.

For now, every one of the "actions" conflicts: accept, reject, resign, claim, close, commandeer, etc, etc. I couldn't come up with any combinations that it seems like users are reasonably likely to want to try, and we haven't received combo-action requests in the past that I can recall.

Test Plan:

• Selected "Accept", then selected "Reject". One replaced the other.
• Selected "Accept", then selected "Change Subscribers". Both co-existed happily.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17132