Worldwide offices

New feature and code for worldwide offices.

We decided not to implement worldwide offices as organisations, but instead make them a first class object and extract functionality from organisations and share it as appropriate. I've done this for social media and for contact information: ideally the same process should be followed for associations to world locations etc.

The code as written is ready to be merged: all the tests pass, the frontend controllers are underneath the /world/offices/ prefix and are therefore protected from the public, and there is no link in the admin from the top menu to get to the admin pages. Visit /government/admin/worldwide_offices to start playing with the functionality.

There is a migration for the `contacts` table which does a part rename of a column: it adds a new column and copies the data over, but leaves the old column intact. A follow up migration will be needed to remove the `organisation_id` column on the `contacts` table.