[keyserver] Don't return thread link to clients
Summary:
We aren't going to display the thread links in the UI and current clients can handle only a single link per one community so it doesn't make sense to return the links to clients.
Depends on D12392
Test Plan: Created a thread link and verified that only community links were returned to the web client
Reviewers: inka, ginsu
Reviewed By: ginsu
Subscribers: ashoat
Differential Revision: https://phab.comm.dev/D12393