Add redirect to HTTPS option

Summary: Rehash of D3411. In cgi/fcgi setups we have no idea if the request is HTTP or HTTPS as far as I can tell, so make this config-triggered again. Also handle @vrana's "off" case.

Test Plan: Set this flag, observed redirect to https when $_SERVER['HTTPS'] was absent.

Reviewers: nh, vrana

Reviewed By: nh

CC: aran

Differential Revision: https://secure.phabricator.com/D3420