[web] Add subchannels button

Summary:
Add a button that will be used in community drawer for navigating to chats subchannels modal.
Related issue
The same button on native: D6001

Test Plan: Added button to ChatThreadListItem and checked that pressing it opens the subchannels modal

Reviewers: przemek, michal, ginsu, tomek

Reviewed By: przemek, tomek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6159