[web] make MessageReactionsModal scrollable

Summary: This fixes and addresses https://linear.app/comm/issue/ENG-3853/the-message-reactions-modal-should-be-scrollable-with-a-bunch-of

Test Plan:
Please see demo video

Reviewers: atul, kamil

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8862