organisation: lessen the greed of network group urls

Network group pages need to have subpages which are not accessible
due to the country url regular expression being too greedy. For it
to be less greedy we have to reduce the regular expression to a
list generated by django_countries (resulting in a large regular
expression).
Merge pull request #180 from okfn/network-subpages

Lessen the greed of network group urls