Trust the user when they say that their server name starts with 'https'

We have a long-standing bug where someone would configure Review Board to use
HTTPS only, but due to proxies or other methods, the HttpRequest wouldn't think
it was https. This meant that any URLs created by the Django resolvers would
start with 'http' instead of 'https'.

Because we know what the user configured their server name as, we can choose in
load_site_config() whether to set the 'HTTPS' environment variable or not. We
also need to set wsgi.url_scheme to 'https' in request.META for wsgi-based
clients.

Testing done:
Configured my devserver with a server name starting with "https" and fetched
the API root. Saw that all the links returned started with 'https'.

Fixes bugs 3085, 3246.

Reviewed at https://reviews.reviewboard.org/r/5490/
Merge branch 'release-1.7.x'