Added ethnicity and race features to the Person model (1.2.0)

`Person`'s within our system need to have the ability to have race and/or ethnicity stored on them. Because it's possible for people to have and identify with multiple races or ethnicities, these fields need to be ManyToMany representations.

This pull request represents one of the rare moments where `tx_people` needs to branch away from the Popolo spec. A [discussion on this](https://github.com/opennorth/popolo-spec/issues/42) lead to the creation of a `national_identify` field, but that doesn't sufficiently solve our problem. The discussion on ethnicity was tabled.

This currently is a blocker on our progress in multiple data apps. If we are going to go forward with our loose adherence to the Popolo spec, this needs to be addressed sooner rather than later.