[keyserver] Add COMMUNITY_[ANNOUNCEMENT_]ROOT to newThreadRequestInputValidator

Summary: This change allows us to create new COMMUNITY_ROOT and COMMUNITY_ANNOUNCEMENT_ROOT threads via the create_thread endpoint. Based on what I've read, I think that should be fine, but please let me know if there's some context I'm missing.

Test Plan:
Able to create COMMUNITY_ANNOUNCEMENT_ROOT as expected:

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7784