[keyserver] Modify thread join responder to handle thread invite links

Summary:
We allow joining a thread when a secret is matched for a community or a thread.

https://linear.app/comm/issue/ENG-8416/modify-thread-join-responder-to-handle-thread-invite-links

Depends on D12393

Test Plan: Tested with the rest of the stack: used an invite link to a thread and verified that the thread was successfully joined.

Reviewers: inka, ginsu

Reviewed By: ginsu

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D12399