Add SSL-redirect middleware to better support all-SSL sites

[Carl Meyer]

Since you pretty much shouldn't do anything with sessions or logins on a public site without SSL, I think a solid majority of public Django sites probably ought to be all-SSL. Given this, I think Django core should provide good support for all-SSL sites out of the box.

One piece of an all-SSL site is redirecting non-SSL requests to SSL. Often this will be done via a front-end proxy server, but a Django middleware to do it is quite simple, and I don't see much reason not to include one.

There's an existing implementation in ​django-secure.

[Sasha Romijn]

This was fixed in 52ef6a47269a455113d95992f868939131f9c10c as part of #17101.