[native] Add subchannels button

Summary:
Linear issue: https://linear.app/comm/issue/ENG-2284/add-links-to-subchannels-in-level-2-chats
Add a button that will be used in the community drawer to navigate to the subchannels modal

Test Plan: Added it to every community item and saw that pressing it navigates to the subchannels modal

Reviewers: ginsu, bartek, tomek

Reviewed By: tomek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6001