libravatar for the BTS (and boring encoding fixes)

While working on fixing a few encoding problems that I managed to introduce to the BTS almost half a year ago, I took a side bit of coding, and introduced libravatar support to the BTS. Every e-mail now has an avatar to the right which should correspond to the sender. Libravatar is a federated service, which means that if you control your domain, you can serve your own icons. It also automatically falls back to gravatar, so if you're using that service, things should "just work". Hopefully this will be primarily amusing, and people won't abuse it.

More importantly, but much less fun, the double encoding problems (where mails would get double-encoded if any of the headers contained non-us-ascii text), and mojibake wontfix icon (☹) should be fixed now. If you see any additional cases of this, please report them to owner@bugs.debian.org.