Start using admin gem

* Add govuk_admin_template
* Stop using less
* Remove jquery-rails, jQuery is provided via gem
* Stop using branches for formtastic, and use Bootstrap 3 versions
Switch to using govuk_admin_template layout

* Use nested layouts
* Make application.css SASS friendly, and import gem styles
* Remove now unnecessary javascript includes
Update to Bootstrap 3 grid classes
Put back bootstrap tweaks and overrides

* Switch from less to SASS
* Delete navbar less file, all styles are now in admin gem
Switch to Bootstrap 3 glyphicons
Buttons need a default class in Bootstrap 3