Each imported row can have multiple legacy urls

The new detailed guides we will import combine multiple source pages
into one single detailed guide.
Failed date parsing generates error not warning

Also I think testing using a mock is nicer.
General purpose finder for any model with slug attribute

Generally we want the same behaviour when finding any model by slug.
Tighten up policies finder
Add detailed guide importer
Test that import fails on unrecognised policy slugs
Nicer slugged model finder

- take logger in constructor, it's a dependency
- don't need line number, we don't use it
- don't use splat args, it makes errors harder to spot
Correct success counts when there are multiple doc sources

It was displaying a successful count of 2 if a single document had 2
document sources.
Merge pull request #301 from alphagov/detailed-guide-importer

Detailed guide importer

https://www.pivotaltracker.com/story/show/44874991