[lib] Add support for thick threads to baseCreatePendingSidebar

Summary:
We'll use a different thread type for sidebars in thick threads.

Additional changes will be necessary in createPendingThread, but I'll handle those separately in a later diff.

Depends on D12489

Test Plan: Flow

Reviewers: kamil, marcin

Reviewed By: kamil

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12490