Add a method to return a base_path for a tag

This adds a method to the Tag class which builds the 'base path' for a 
tag instance. The method concats the slug, and the parent's slug (where
present) to build the path which the page would live on GOV.UK.

This method can be overridden by subclasses to provide their own 
prefix, as has been done with the MainstreamBrowsePage class to add the
'/browse' prefix.
Improve the layout of the browse page interface

This includes a handful of changes to the layout of the interface for 
managing browse pages, simplifying some of the markup and styling of
list pages and making forms look more consistent with other GOV.UK admin
applications.