Switch to using govuk_admin_template

* Use styles, templates and JavaScript provided by gem
* Delete styles, templates and JavaScript that have been ported
* Rename GOVUK to GOVUKAdmin in JavaScript to match gem
* Remove bootstrap and jquery-rails dependencies, now served through gem
* Create a new transition_theme stylesheet for variables not appropriate for gem
* Put tag styles into their own component stylesheet
* Mount gem's /style-guide route