Add footer pointing to onion address

It would be nice if we could add the onion address to the footer, like we do on https://account.riseup.net, to encourage users to use tor.

Probably we should use the same format we do on https://account.riseup.net (although the onion address should be http://zkdppoahhqu5ihjqd4qqvyfd2bm4wejrhjosim67t6yopl77jitg2nad.onion instead).

@elijah had an idea of making a gem with the onion addresses, so we could just include them in our rails apps and only update them in one spot, but until we have that, maybe a simple footer change could be done to include this on the login page, and the common footer?