Code to update Boundary Line for October 2013

Includes: 
- A 2013-10 control file which overrides area creation for Northumberland, Gateshead, Stevenage and East Hertfordshire, so that we don't lose their IDs.
- A management command to manually fix the GSS codes and generations of the overriden areas above, and re-import the old boundaries they represented under new id's in the older generations.
- A fix for `mapit_UK_import_boundary_line` when looking up areas by unit_id, because that's not always unique when we've manually preserved old areas: https://github.com/mysociety/mapit/commit/aed3d9054ba91163ea22fe5b0699f415e36fbd2a