Replace links in navigation

Includes:
- removing all styling of those links
  outside of GOV.UK Frontend (except for a few fixes
  due to their interaction with our design)
- bringing together some duplicate styles into one
  block
- changing how links are marked as selected, now
  they have multiple classes

Fix Sass-lint error in navigation.scss

app/templates/org_template.html

@@ -10,7 +10,7 @@
       {% if current_user.platform_admin %}
         <a href="{{ url_for('.organisations') }}" class="govuk-link govuk-link--no-visited-state navigation-organisation-link">All organisations</a>
       {% endif %}
-      <div class="navigation-service-name">
+      <div class="navigation-service-name govuk-!-font-weight-bold">
         {{ current_org.name }}
       </div>
       <a href="{{ url_for('main.choose_account') }}" class="govuk-link govuk-link--no-visited-state navigation-service-switch">Switch service</a>