Uc 115 abstracting blacklist of redirect locations after facebook signup

Don't merge yet

The original bug was that redirecting from pages like Special:Signup after facebook signup wasn't working. However, we don't actually want to redirect back to pages like this. 

This PR takes the same logic used in SpecialFacebookConnect and uses it for facebook signups via modal. 
Added context binding so this `UserLoginAjaxForm` instance will be available to the callback function.