Add style guide (from govuk_admin_template)

This allows previewing how template styles mix with the application styles.
The style guide is available under `/style-guide`.

The change to the application helper fixes a bug caused by `current_page?`
being unable to deal with having both a module called `AnonymousFeedback`
and a controller called `AnonymousFeedbackController`.
Merge pull request #194 from alphagov/add-style-guide

Add style guide (from govuk_admin_template)