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:
{F850779}
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T9479
Differential Revision: https://secure.phabricator.com/D14186