Adjust language of LMS emails to work specifically for Stanford. Here
there be dragons: lots of ugly conditionals that will need to be
changed once we develop a way to "theme" arbitrary strings throughout
the site.

Instead of hardcoding things like the platform name, use the
corresponding overrideable settings instead. This allows themes to
control emails as well.

Moved site, is_secure, etc. into render_to_string shortcut, and fixed bugs in the shortcut. We should test this pretty well, since this touches a lot of templates