fix: fixes #392

[gregorpirgie]

Replaces fetching instances

• from using get_object_from_pk_or_uri
• to using get_object_or_404

This adds an automatic check for each endpoint if requested object is indeed instance of expected entity. Otherwise, redirects to 404 page.

Removes unused import.

[gregorpirgie]

Updated solution, see latest commit.

But instead of throwing 404, returns a HttpResponse with error message in plain text (thought this would be better for programmatic interaction with the endpoint(s)).