Add api for just getting a geojson of a district

damn, son. here's some geojson.

#### testing:
* test suite
* try out the geojson in geojson.io
* one way to get a sample geojson is to install this into the texastribune project and hit localhost:8000/lege-districts/api/district/house/5/

#### code smells
double namespace? whaaa? Well at first I was just trying to make sure the `district_detail` wasn't confused for a html district detail page. So I started throwing `_geojson` on, then I tried `_json`, then `_api`, then `api_`, then I realized I was just trying to namespace the url. So I used a namespace.


#### future
I think there should be a at least two urls to get the same shape: 1) a cannonical one here, and in the future: 2) a shortcut human friendly one like /cd-5/ instead of /congress/5/