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.
Really minor nit, but you could just use:

config.setFallbacks(Arrays.asList(fallbackProxy));

to shave off a couple lines.