add force redirect from HTTP to HTTPS

[lmccart]

http://www.paulirish.com/2010/the-protocol-relative-url/

[toolness]

Just so you know, HTTPS breaks the weather example. :disappointed:

A current workaround is to access that example over HTTP, but forcing the redirect would make that untenable. (That particular example is also non-trivial to fix because the free version of the OpenWeatherMap API only works over HTTP. :sob: )

I'm not sure if there are any other examples that fetch resources from other sites over HTTP, but those might break too.

Also, if anyone wants to tinker with the example snippets in ways that might cause mixed-content warnings when p5js.org is on HTTPS, then their tinkering would fail as well. We ran into this kind of problem a lot when Thimble was hosted over HTTPS, and I think it's part of why a lot of tinkering sites like JS Bin and CodePen remain served over HTTP to this day (though I could be wrong).