Add authentication using Persona (BrowserID)

To get the BrowserID JavaScript include not to conflict with the app,
I had to remove JQuery from the Require.js config and load it directly
from HTML.

Currently it behaves like this:

ADMIN_EMAIL set => Must login successfully with that email
ADMIN_EMAIL unset (default) => Anyone with a Persona ID can log in

Static files (/static/*), including hotlinks to music files and cover
art, are currently not access-controlled. This will probably be worth
fixing at some point.