Uc 115 abstracting blacklist of redirect locations after facebook signup

Note: This PR points to uconn-1 branch so we may want to create a new PR to point to the uconn-2 branch. 

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. 

We should probably add a test or two for Facebook Signup at some point .
Where is `this.returnToUrl` populated?