[MAKE-1581] Block free email domains from signing up

@StephenCharles @maj 

## Overview
* We want to keep people with a free email address from signing up for an account

## Related Links
* [[MAKE-1581](https://crowdflower.atlassian.net/browse/MAKE-1581?filter=-1)
It's a bit strange this gem tests the validations this way. As most gems use the 'dummy' app specs.