[keyserver] Extend link validation to support thread links
Summary:
In addition to the previous solution, fetch thread-related data and return the response based on both community and thread data.
Depends on D12354
Test Plan:
Checked returned verification status for community and thread links when viewer is and isn't a member.
Also checked if joining a thread link results in joining a community.
These two checks additionally validate that the older client will work correctly with the new solution to links.
Reviewers: inka, ginsu
Reviewed By: ginsu
Subscribers: ashoat
Differential Revision: https://phab.comm.dev/D12392