[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.

https://linear.app/comm/issue/ENG-8412/modify-fetch-invite-links-endpoint-to-return-only-community-links

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