Add favicon to website

It'll be nice to see the Briar logo on the browser tab. That should help people easily identify a tab with Briar among many other browser tabs.

This should be as easy as adding the following withing the head:

<link rel="icon" type="image/x-icon" href="link-to-favicon.ico">
<link rel="icon" type="image/png" href="link-to-favicon.png">

While doing this, change the title from "Secure messaging, anywhere - Briar" to "Briar - Secure messaging, anywhere".