Persona 834279

This has persona support plus a couple of other nitty things that I changed.

I'm pretty sure this is right, but there are a few things I wasn't positive about:

1. django-browserid has a js file that needs to be included. This does it in a way that pulls it from django-browserid lib, but it's not in the minify bundle. I'm pretty sure that's wrong, but I don't know offhand how to fix it.

2. It adds a "Sign in" link to the header but I think that suggests that if you sign in you get special things. Right now the only people who can sign in are people who explicitly have accounts which we'll be limiting to admin/superusers. Given that, maybe the link should be less prominent? Having said that, this will change possibly in the next few months.

3. The text on the login failure page is off the top of my head. Someone should look at that and/or compare it to what other persona-enabled sites do.

I think that's it. It's using master tip of django-browserid since I told @osmose that I'd help him test things. Figured we're a good testbed.

r?