[web] Make chat mentions clickable

Summary:
Solution for ENG-4561.

This diff uses useOnClickThread hook which redirects to chat when mention is clicked and uses it in markdown-chat-mention.react.js

Depends on D8852.

Test Plan:
Before testing, please apply the entire web stack.

Test if mentions inside a spoiler are properly rendered and are clickable only when spoiler is opened:

Reviewers: tomek, inka, rohan

Reviewed By: inka

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8875