fixes 1033849 - debug persona login

@rhelmer (cc @AdrianGaudebert) r?

It's not pretty but it's potentially super useful to people struggling with login.
![screenshot 2014-07-02 16 59 42](https://cloud.githubusercontent.com/assets/26739/3464040/006594b0-0245-11e4-8da6-43c99fe00b2c.png)

Having the same view be the source of the first HTML data _and_ of the following JSON data is very confusing to me. Is it something usual in django? I don't remember seeing this before.