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>
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