R1:a2a0f002f069

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…