Unify permissions for community OPEN and SECRET subthread to solve the issue:
https://linear.app/comm/issue/ENG-1004/error-when-adding-user-that-is-not-in-parent-thread-to-open-subchannel

The diff requires database migration to fix permissions in already existing threads. It will be introduced in follow-up diff.

Create new Open thread nested in Secret thread - the issue described in ENG-1004 should no longer exist.
Users not in parent thread can now be added to child thread. Tested in mobile and web app.
After database migration the issue should also disappear in all already existing threads.