Adds the ability to filter Artefacts by owning application

* Refactors the exclusion of panopticon owned applications for easier testing

(Paired with @JordanHatch)
Oh gosh. Sorry.

I think that test is superseded by the new refactoring of panopticon exclusion in:

* https://github.com/alphagov/panopticon/pull/175/files#diff-2a3e7e1de3019253ae92099894bae72cR78, 
* https://github.com/alphagov/panopticon/pull/175/files#diff-2a3e7e1de3019253ae92099894bae72cR84
* https://github.com/alphagov/panopticon/pull/175/files#diff-9b42dbaf2f6abccf7dfdfc4ee6662460R123

(CC @JordanHatch - I think that test can be removed, the behaviour it's testing is fully covered by smaller, simpler tests - agree?)