Move us over to https!

Summary:
According to
   http://phabricator.khanacademy.org/config/edit/security.require-https/
we may also need to set up nginx to pass through the HTTPS variable
properly.  I'll do that before landing this.

Don't set security.require-https after all, but do the redirect in nginx.

Test Plan:
Fingers crossed!  I don't know how to test it without just doing it on
our live site.

None

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D14779