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