[web/lib] introduce MessageReactionsModal

Summary:
introduce MessageReactionsModal. MessageReactionsModal shows all the users who have reacted to a message, what their username is and what reaction they used. I decided to punt the implemenation of the filter buttons at the top of the design since every message will be a like for the moment

Depends on D6301
Linear Task: ENG-2718
Figma

Test Plan:
Please watch the demo to see how MessageReactionsModal works

Reviewers: atul, tomek, ashoat

Reviewed By: tomek, ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6302