Extracted document mapping export to class

This is in preparation for adding tests around the mapping extraction
process.
Regression for publication extraction

Ensured that publications are correctly extracted to the
document_mappings file. This is a simple regression as a prelude to
adding further tests for new functionality.
Added corporate information pages to mappings
No admin 'show' pages for corporate info
Merge pull request #259 from alphagov/corporate-information-pages-in-mappings

Corporate information pages in mappings
Fix Unable to find News article type error message
Remove unused search configuration variables

We never search the mainstream or detailed guidance index from within
whitehall. Searching these indexes is handled by the frontend app.
Fake GdsApi::Rummager not DocumentFilter

Faking the behaviour of DocumentFilter means that we don't test an
important part of the implementation of our filter pages. This change
means that all of our tests do exercise our real filter
implementation(s). This feels like a better cut-point at which to
introduce the fake behaviour.

Looking forward, it feels as though the fake implementation really
belongs in the same package as Rummageable and GdsApi::Rummager, but
these classes are in different packages from each other at the moment,
so we can't do that yet.

I've also changed NotQuiteAsFakeSearch::Store to not be a singleton, but
rather be passed in the constructor explicitly to the objects which
collaborate with it. I think this explicity dependency-passing style
makes the relationships between objects more clear and explicit.
Fake rummager should handle nil field values gracefully

By coincidence we hadn't hit this scenario in tests until now.
Don't filter by local government relevance unless requested

The previous implementation would mistakenly hide local government
content if the checkbox was unchecked, and show it if it was checked.

However, the intention is that you could request only local government
content if desired by checking the box, and otherwise you would see
everything.

https://www.pivotaltracker.com/story/show/44806697
Force platform environment variable

This forces the URL generation logic to be consistent across platforms.
Make organisations dynamic

The hard-coded "1" was obviously going to fail in a full environment.