cms: allow iframes to be used in the text plugin

This creates a security concern, so users/editors should be managed
well.
organisation: make network group lists more independent

This allows users to select where they want chapters vs local
groups instead of the one where they just get added one after
the other. Uses the same plugin but gives user the choice of
chapter vs local group
organiation: update cms plugin tests for new network group flag plugin

Since the network group plugin now allows users to choose group type
tests have to be updated.
Merge pull request #163 from okfn/network-group-fixes

Network group fixes