[lib] Move RegExp used to bold @-mentions to mention-utils.js

Summary:
This diff should be a no-op. Just moving things around, so all of the @-mention RegExps are in one place.

Depends on D6761

Test Plan: Flow

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6762