Rake task to import orgs from GDS API

Also adding Importer and spec test for importer
Contact now uses Organisation, removes Department

Also changing all the tests for the models and associated bits of non-view
or controller that touch contact.

Part of https://www.pivotaltracker.com/story/show/67487248
Contacts uses Organisation in Views

Also changing all controllers, feature tests and routes
Add index for organisation_id on contact_groups
Test cleanup for using Organisations

Some tests from Master were now failing with my new Organisations tests.
This commit fixes those, namely view_contact_spec and view_contacts_spec.

Also changing how Groups are matched to Organisations in seed by doing a
find or create in seeds and removing the old call for the orgs API in env
files.
Refactor Organisation spec to test methods

As per:
https://github.com/alphagov/hmrc-contacts/pull/113#commitcomment-5908997
I've refactored this test to test the actual methods in Organisation.rb