Don't allow sorting on geo_point

In discover, if you add a field of type `geo_point` to your search, and then try to sort on that field, the app errors out. It doesn't make sense to allow sorting on geo_point as this is not supported by ElasticSearch. Perhaps in the future we can allow sorting `geo_point` fields by distance from a specified location.

Closes #1933.
What if we changed `mapping.type !== 'geo_point'` to `!_.contains(unsortableTypes, mapping.type)` and then add a variable above with an array of the un-sortable types... then we can add more later.