wooha. With some caveats, the sign up flow where authentication with the primary is working.

* Use WinChan to open a new window to the primary.
* Add the idp_auth_complete page for the IdP to redirect back to.
* When the window closes, re-try to authenticate the user with the primary.
* Add a helper to show an error message.
* Add a WinChan mock.
* Update unit tests to handle the "need to authenticate with IdP" scenario.
* Update compression scripts for WinChan to be included on the main site.