[keyserver] Fix keyserver to allow adding non-friends from the same community to thread

Summary: Introduce fix for following issue: https://linear.app/comm/issue/ENG-1223/fix-keyserver-to-allow-adding-non-friends-from-the-same-community-to

Test Plan: Created community chat in Comm and tested cases for friend/non-friends into community subchannel/community root/sidebar. It should now be possible to add only friends to community, and anyone already who is in community (even non-friend) to the community subchannel.

Reviewers: tomek, atul, ashoat

Reviewed By: ashoat

Subscribers: ashoat, adrian, abosh

Differential Revision: https://phab.comm.dev/D4710