R1:fa943f744bc4

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…