[bug 834279] Add persona authentication

* add persona support for authentication. it's set up such that it doesn't
  create new accounts--accounts must be created manually by an admin.
* put browserid.js file in base bundle
* change the dashboard templates to inherit the site_js block
  from base.html
* attempt to add comments that explain what's going on so it makes
  sense to future developrs
* add documentation to help new people set persona
* tweak navbar to add a section on the right
Nix unused import
Add TODO for removing stubs