[lib] Turn on notifs by default for thick threads

Summary:
When I first introduced this code, I used defaultThreadSubscription, thinking it corresponding to having notifs turned on.

After testing as part of ENG-8426, I found that thick threads were being created with notifs turned off. This made me realize I should be using joinThreadSubscription here to make sure that the notifs are turned on by default for thick threads.

Depends on D12824

Test Plan: I followed the test plan described in this Linear comment. Before this diff, the thick thread was created in the Muted tab. After this diff, it now appears in the Home tab

Reviewers: tomek, kamil, marcin

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D12825