Stop all object mentions from matching after "@"

Summary:
Fixes T9479. Currently, @aaaaaaaa may try to match as a commit hash, and @C123456 may try to match as a Countdown reference. These should only match as user mentions.

Prevent object mention rules from matching after @. We already prevent them after - and #, and already prevented the username rule after @ (i.e., preventing @@user).

Test Plan:
Created some "interesting" users locally and @mentioned them:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9479

Differential Revision: https://secure.phabricator.com/D14186