Create JSON endpoint for Service and Genre list and detail views

[dchiller]

Returns JSON responses to requests to Service and Genre urls if the request's Accept header is application/json.

Implements a new class JSONResponseMixin that handles the content negotiation according to the Accept header. Classes inheriting from this mixin should define a json_fields attribute that lists the fields on the model that should be returned in the JSON response.

JSON responses are in the form of a object with a key being the model name (singular, in the case of a DetailView and plural in the case of a ListView -- this matches the CantusDB standard for urls).

For example, a request to /services with Accept: application/json will return:

{
  services : [
    { object with json_fields for service 1 },
    {  object with json_fields for service 2 },
  ]
}

A request to /service/1234 with Accept: application/json will return:

{
  service : { object with json_fields for service 1 }
}

The PR also adds appropriate tests for the GenreDetail, GenreList, ServiceDetail, and ServiceList views. These tests weren't really making use of Django's TestCase functionality for setting up test data, so I did some refactoring there too.

Finally, the GenreListView was inheriting from a SearchableListMixin but was not making use of that mixin's functionality (the filtering for that view was implemented in the overwritten get_queryset method), so I removed that inheritance.

Closes #1609.