[keyserver] Block creating new chats with users that you are not friends with
Summary:
Users could create a chat with non-friends because every chat's parent is genesis. Added special case for genesis.
Test Plan:
Tested creating new threads in genesis:
- user with friend + nonfriend -> blocked,
- user with friend + friend -> ok,
- user with nonfriend private -> ok.
Reviewers: tomek, michal, inka, kamil
Reviewed By: michal, inka
Subscribers: ashoat, atul
Differential Revision: https://phab.comm.dev/D6744