[lib] Leave thread sidebars while leaving the parent thread

Summary:
When a user leaves thick non-sidebar thread, we're deleting it. We should do the same with all its sidebars.

https://linear.app/comm/issue/ENG-9098/update-thread-leaving-logic

Test Plan: Created a thread and a sidebar. Left the thread and verified that the sidebar is no longer in the store.

Reviewers: kamil, inka

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13291