Add styling for specialist sector browse pages

This adds govuk_frontend_toolkit, includes multiple mixins into the
application.scss manifest, and adds a stylesheet for specialist sector
browse.

The specialist sector browse styling is largely based on the existing
style rules from the Frontend app, however I have simplified the CSS
in many places to remove rules which were specific to other browse
pages. This should make it easier to change in the future.
Set Content API web URLs relative to the website root

Define a custom instance of the Content API adapter, configured to use
the value returned from 'Plek.current.website_root' as the hostname
for the tag URLs. This fixes tag URLs in development pointing to the
'www' host.