Initially we were assuming that all the users that click an invite link are logged in to a keyserver. In multi-keyserver world it can happen that a user is logged in to Identity service, but isn't to a keyserver.
Before logging the user in, we need to check if a link is valid - that requires modifying the verification endpoint so that it no longer assumes that a user is logged in. This assumption was used only to determine if a user is a member of a thread - which is obviously false for a user that isn't logged in.
Depends on D10307