Several use cases for handling custom domains over SSL.

- Force SSL during login and admin sessions.

- New Wordpress instances should be available over SSL and non-SSL.
  i.e. Both https://foo.herokuapp.com and http://foo.herokuapp.com
       should work as expected.

- Custom domains should be available over SSL and non-SSL.
  i.e. Both https://foo.com and http://foo.com should work as
       expected. (N.B. You may receive a browser warning if the
       SSL cert is misconfigured.)

- Update nginx.conf: reject /favicon & /robots.txt requests