Opendownload

This incorporates changes to the email templates for invitation emails to list mirrors instead of user-specific downloads.  Right now [GitHub](https://github.com/getlantern/lantern/releases/tag/latest) is the only mirror, but we can easily add others.

We'll need to pull translations eventually once they're available.
Actually, having a download page on getlantern.org might look nicer.  If someone can get to getlantern.org, might as well give them a better styled experience.