Add count parameter to media endpoint

[RayceeM]

Fixes issue #1664

[ukanga]

I think the appropriate way to add non standard headers would be a header that starts with X-. Seeing the need to have a total count makes me think we probably should implement pagination properly as there are different ways this information could have been exposed. See https://www.django-rest-framework.org/api-guide/pagination/ for more information. We also have the StandardPageNumberPagination class in https://github.com/onaio/onadata/blob/master/onadata/libs/pagination.py#L4.

[ukanga]

I think it is best we rely on using self.filter_queryset(self.get_queryset()) so that we maintain the queryset and filters to applied to it by the viewset. Hence

data = {
  "count":  self.filter_queryset(self.get_queryset()).count()
}

return Response(data=data)