Fix security.require-https by marking redirect as external

Summary:
Resolves T5937. HTTPS redirects caused by security.require-https use a full scheme, domain and port in the URI. Consequently, this causes invocation of the new external redirect logic and prevents redirection from occurring properly when accessing the HTTP version of Phabricator that has security.require-https turned on.

I've also fixed the automatic slash redirection logic to add the external flag where appropriate.

Test Plan: Configured SSL on my local machine and turned on security.require-https. Observed the "Refusing to redirect" exception on master, while the redirect completed successfully with this patch.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5937

Differential Revision: https://secure.phabricator.com/D10318