Ready for QA: Split sign up and sign in when accepting invitations

Just wanted to get feedback on this feature. As it stands, when you click the 'start a new group' link in your email, you will get directed to one of these two pages, depending on whether or not your email address is recognised:

![screen shot 2013-07-12 at 11 33 32 am](https://f.cloud.github.com/assets/970124/785510/6f7f92d2-ea82-11e2-8eac-8aaa8679afee.png)
![screen shot 2013-07-12 at 11 30 26 am](https://f.cloud.github.com/assets/970124/785511/6f957b60-ea82-11e2-9974-579ce7eba2a7.png)