404 should be a 400 for invalid URL

* Also, fix quoting consistency for non-interpolated strings
Merge pull request #198 from alphagov/fix-global-find-error

404 should be 400 for invalid URL
Refactor step to make it parameterised

I'm about to use it to create two sites in one scenario.
Scope tags autocomplete to the site
Merge pull request #195 from alphagov/make_tags_autocomplete_scoped_to_site

Scope tags autocomplete to the site