Add a mixin to ease the amount of code and imports required for views that extract the content type from URL parameters. Improve ExternalObjectViewMixin by adding a new class attribute "external_object_pk_url_kwargs" to mechanize URL parameter extraction. The external_object_pk_url_kwargs maps model attributes using during manager get or filter from URL parameters. Signed-off-by: Roberto Rosario <Roberto.Rosario@mayan-edms.com>