[web] Add chat mention rule to text message rules
Summary:
This diff adds chat mention rule to markdown rules file. Raw chat mentions should be replaced with MarkdownChatMention component.
Depends on D8876.
Test Plan:
Please see test plans in diffs listed below:
Reviewers: tomek, inka, rohan
Reviewed By: rohan
Subscribers: ashoat, tomek
Differential Revision: https://phab.comm.dev/D8853