XGENTOOLS-1070: Onboarding modal

This changeset renders a modal popup for first time MMS docs visitors, asking them to specify what version of MMS they're using. The user gets redirected to the correct url if they belong somewhere else and their choice gets saved as a cookie.

![screen shot 2014-05-14 at 3 26 40 pm](https://cloud.githubusercontent.com/assets/202157/2976189/b52c9fde-db9d-11e3-87d0-fecf922741e8.png)

Users can still change their setting in the option panel in the bottom left.

![screen shot 2014-05-14 at 3 23 49 pm](https://cloud.githubusercontent.com/assets/202157/2976161/70905b86-db9d-11e3-921a-77a41940ab2e.png)