[native] introduce thread notif settings notice text

Summary:
In our thread notif settings we display some notice text + disable the background option if the thread is a sidebar

Depends on D12648

Test Plan:
Please see the screenshot below

Reviewers: ashoat, inka

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12662