[web] introduce MarkdownUserMention component

Summary:
This diff makes the @mention of a user interactive with the introduction of this component. When a user clicks on this component, the user profile modal will be pushed

Linear Task: https://linear.app/comm/issue/ENG-5245/make-user-profile-accessible-from-markdown-user-mention

Depends on D9486

Test Plan:
Please see the demo video below:

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9488