Auth: add a cookie to pin user to beta instance

[agjohnson]

On login or maybe on use of the beta dashboard, we should set a cookie or session variable to redirect the user to the beta dashboard whenever they log in. The notification links will be temporary

[humitos]

On login or maybe on use of the beta dashboard

I think this won't work since we need the beta=true cookie to be on readthedocs.org not on the beta dashboard domain. However, I suppose that we can set the cookie when the user clicks on the notification to go to the new dashboard from the old dashboard

and remove when they click on the notification from the new dashboard.

I suppose we will need to modify that link with something like readthedocs.org/?cookie=remove and remove the cookie from Django since it's also a different domain.