Add missing nullable constraints

I had to change the factories and tests to be able to generate valid data.

Conflicts:

	app/models/organisation.rb
	db/structure.sql
	spec/models/organisation_spec.rb
Deal with shoulda failure on NOT NULL column

See https://github.com/thoughtbot/shoulda-matchers/issues/194
Specs no longer get away with null orgs

* Add an org for SiteYamlFile tests to link to
Merge pull request #109 from alphagov/not-nullable-constraints-redux

Not nullable constraints redux