Limit area lookups by unit_id to the current generation.

There can be areas from previous generations which share the same unit_id but which are different. EG: http://mapit.mysociety.org/area/143398.html replaced http://mapit.mysociety.org/area/15941.html but kept the same unit_id. Without this additional lookup we don't know which one to check
Add control file for October 2013 boundary line changes.

Some manual workarounds needed for boundaries which have got new GSS codes but we don't want new areas to be created for them.
Add a management command to fixup Oct 2013 Boundary Line Changes.

Restores the areas we overrode when importing the Oct 2013 Boundary Line and fixes up the new areas to have the right GSS codes and generations.