Add models for representating relationships between organisations

We need this for representing the link between constituency
offices and parties, but it would also have been useful for
representing party -> coalition relationships around the
Kenyan election.
Increase the max length of the Place slug to 200

This makes it consistent with the lengths of the other slugs
in the models file and allows us to store some consituency office
place slugs that end up longer than the previous max length (100).
A more sensible __unicode__ method for identifier
Remove a duplication of the Organisation on a Place page
South Africa: add a script to import constituency office data

This is difficult data to deal with, and as a result this
one-off script is rather horrible.  (It relies on having the
'all constituencies2.csv' file provided by Geoffrey Kilpin.)

Please bear in mind the caveats mentioned in the comments at
the top of the script.
Merge branch 'constituency-office' into south-africa