Use header component for masthead

Replaces the following blocks/variables with the
`header` block filled by the `header` component:
- header_class
- proposition_header
- global_header_text
- homepage_url

Also rewrites code that selects header nav items

The original code inserts a class name to mark the
nav item as selected.

The GOVUK Frontend header just needs a boolean to
be passed to indicate an item is selected.

This uses the `header_navigation.is_selected`
method, as before, but changes its return value to
a boolean.

app/navigation.py

@@ -328,6 +328,12 @@ class HeaderNavigation(Navigation):
         'whitelist',
     }
 
+    # header HTML now comes from GOVUK Frontend so requires a boolean, not an attribute
+    def is_selected(self, navigation_item):
+        if request.endpoint in self.mapping[navigation_item]:
+            return True
+        return False
+
 
 class MainNavigation(Navigation):