[native] style SidebarListModal, SubchannelsModal, ThreadListModal

Summary:
Linear issues: https://linear.app/comm/issue/ENG-2547/restyle-sidebarlistmodal-and-subchannelslistmodal, https://linear.app/comm/issue/DES-20/designs-for-subchannels-modal
SidebarListModal, SubchannelsModal use ThreadListModal undeneath, and no other component uses ThreadListModal. SidebarListModal and SubchannelsModal are supposed to look consistent, and match the new designs provided in the second linked issue.

Test Plan: Run ios simulator, saw that the two modals look as expected. Chceked that scrolling works poperly.

Reviewers: kamil, tomek, michal, przemek, ashoat

Reviewed By: tomek, ashoat

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6073