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/navigation.py

@@ -7,7 +7,7 @@ class Navigation:
 
     mapping = {}
     exclude = {}
-    selected_attribute = "class=selected"
+    selected_class = "selected"
 
     def __init__(self):
         self.mapping = {
@@ -32,7 +32,7 @@ class Navigation:
 
     def is_selected(self, navigation_item):
         if request.endpoint in self.mapping[navigation_item]:
-            return self.selected_attribute
+            return " " + self.selected_class
         return ''
 
     @staticmethod
@@ -42,8 +42,6 @@ class Navigation:
 
 class HeaderNavigation(Navigation):
 
-    selected_attribute = "class=active"
-
     mapping = {
         'support': {
             'bat_phone',