Custom testing: Add request relaying to test.critic-review.org

This is part of the custom testing feature.
Drop "Push to master" functionality

Instead, add a "check if review is ready" operation.  This is used
by the new CriticCollaborator extension, which now implements the
"Push to master" functionality for collaborators.
Style updates
Test a bit more and less

Don't test every commit upgraded from every upstream, instead test
only the commit at the tip of the review branch from every upstream.

Do test the commit at the tip of the review branch upgraded from
every refs/heads/historic/* though.