Add client-side check for protocol mismatch

Summary:
Fixes T10402.
I tried about 50 variations on the wording and notification layout, this seemed by far the most reasonable.
Didn't implement a way to ignore the warning, which might be required - but figured this is serious and broken enough while being completely invisible 99% of the time that it's worth shouting about.

Test Plan: Messed around with $_SERVER['HTTPS'] on the server side and client_uri on the client side - saw reasonable results in all combinations.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Maniphest Tasks: T10402

Differential Revision: https://secure.phabricator.com/D16064