Improve tests ahead of refactoring some of the views

* Re-organised the with_tag tests, and added assertion for some of the fields.
* Added assertion for fields in licences test.
Return fewer details in the with_tag view.

This has involved refactoring the views somewhat:

* the _full_artefact partial include the full details hash, but not related artefacts, and tags - these are now explicitly included in the artefact view
* the with_tag view now uses the basic_artefact partial, and adds the description details key that's needed by the browse pages.
* the licences view also uses the basic_artefact partial, and adds the licence details keys needed by licence-finder.
Get rid of _full_artefact.rabl

It was only used in the artefacts view, so I've inlined it there.
Merge pull request #97 from alphagov/with_tag_performance

Improve performance of /with_tag.json endpoint