[native] Extract SidebarItem

Summary: While working on the web side of this, I realized it's weird for ChatThreadListSidebar to be handling both the ChatThreadList and the SidebarListModal use case, especially since it's basically just a conditional branch.

Test Plan: Flow

Reviewers: KatPo, palys-swm

Reviewed By: KatPo

Subscribers: zrebcu411, Adrian

Differential Revision: https://phabricator.ashoat.com/D453