fixes bug 941358 - 403 when logging in while having a second tab open

@AdrianGaudebert r?

This isn't my ideal of a patch but it's the best we can do. So, current (released) django-browserid can not and will not be able to work with multiple tabs. In the unreleased version of django-browserid, instead of doing `$loginForm.submit()` which will cause a HTTP POST submission it instead does `$.post('/browserid/....`. This is how the future version of Persona will work (aka. The Goldilocks rewrite) which means that if you have two tabs open (one "Sign in" button on each), sign in in one, and then switch over to the other tab you'll have to refresh that tab to see that you are now signed in. 

Because the new django-browserid is patiently waiting for the new Goldilocks rewrite @mkelly doesn't want to make a new release so that's why I'm adding the current head of django-browserid in as a submodule. 

The reason the patch is so big is because I'm upgrading the `requests` lib in vendor-local. The one we had before was really old.