The previous theme test relied on the existence of the tutorial theme, which has been removed.
I'ved added a couple of behat rules for scaffolding fake themes, and altered the test to use these.
Too many clashes with other steps otherwise,
e.g. with 'And the preview contains "You can fill"'.
Also removed some duplicated code no longer necessary
because we use inheritance rather than composition
They can be unpublished through an explicit step.
Extending from base FeatureContext in order to easily
share the same FixtureFactory but still add new blueprints to it.
