Allow asset admin styles to be used in 'Choose another file' dialogue of UploadField

I've added thumbnails to the 'Choose another file' dialogue, this style change allows for that gridfield to be styled the same way with compact rows
Reflect changes in scss file
Behat: Auto-publishing SiteTree fixtures, RedirectorPage fixtures

They can be unpublished through an explicit step.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.
Behat: Replace @database-defaults with explicit fixtures
Behat: Fixed spelling in test
More verbose fixture step definitions

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
Merge remote-tracking branch 'origin/3.0' into 3.1

Conflicts:
	tests/behat/features/edit-a-page.feature
	tests/behat/features/preview-a-page.feature
Behat: Save draft notifications have changed
Behat: Adding page publishing scenarios #616
Behat: Type case in step definition
Behat: Adding user help scenario #607
Behat: New feature "Edit site wide title and tagline" #619
Behat: Fixed indentation (no functional changes)

Using tabs consistently
Behat: Removed unnecessary access-user-help.feature

It doesn't add much value in terms of testing
(can't see how we'd ever mess up the "Help" menu entry),
and it relies on the availability of an external resource
(userhelp.ss.org), making the tests more brittle.

It also breaks the feature following this one, because
somehow Behat doesn't reset the browser session, so
subsequent logins fail because a login still exists.
Merge pull request #761 from jonom/patch-1

FIX #527 : Add Thumbnails to 'Choose another file' dialogue
ENHANCEMENT: allow sites to override meta generator tag