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.
Correct.  The selection of OS/platform happens on the GitHub page.  We should follow this pattern for all mirrors so that we don't end up with an explosion of links.