[keyserver] Extract isMentioned function

Summary:
This diff should be a no-op. I'm not actually going to use this function anywhere else, but I'd like to keep all the @-mention RegExs in one file.

Depends on D6760

Test Plan: Flow

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6761