[keyserver] Validate fetchPrimaryInviteLinksResponder output

Summary:
Add validators for fetchPrimaryInviteLinksResponder.

Depends on D7808

Test Plan: yarn jest-all, try running web and landing and see if they load correctly

Reviewers: kamil, tomek

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7848