[native] Extract from SidebarListModal code that can be reused for the subchannels modal

Summary:
Related Linear task: https://linear.app/comm/issue/ENG-2373/subchannels-view
The subchannels modal is supposed to be similar to SidebarListModal, so I extracted from it the code that can be reused.

Test Plan: Run ios simulator, see that sidebar modal works

Reviewers: przemek, kamil, tomek

Reviewed By: kamil, tomek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D5971