[lib] Fix getPotentialMemberItems to match server logic for adding users to threads in community

Summary:
Adjust getPotentialMemberItems logic to match permissions on server side.
Linear issue: https://linear.app/comm/issue/ENG-1200/creating-thread-with-non-friend-user-is-blocked-on-front-end-and

Test Plan: Tested the change by trying to add friends/non-friends to community root/community [open|secret] subchannel/sidebars. It is possible to add any community member to community subchannel, and only friends to community.

Reviewers: tomek, atul, ashoat

Reviewed By: ashoat

Subscribers: ashoat, adrian, abosh

Differential Revision: https://phab.comm.dev/D4711