Highlight selected item in proposition navigation

It is standard practice when using GOV.UK template to highlight the
selected navigation item in the propositional navigation (black bar) by
colouring it blue.

This commit adds a new subclass of `Navigation` with the mapping needed
to decide which pages belong to which item in the navigation (or none
at all).

app/__init__.py

@@ -39,7 +39,7 @@ from werkzeug.local import LocalProxy
 from app import proxy_fix
 from app.config import configs
 from app.asset_fingerprinter import AssetFingerprinter
-from app.navigation import MainNavigation
+from app.navigation import HeaderNavigation, MainNavigation
 from app.notify_client.service_api_client import ServiceAPIClient
 from app.notify_client.api_key_api_client import ApiKeyApiClient
 from app.notify_client.invite_api_client import InviteApiClient
@@ -90,7 +90,10 @@ current_service = LocalProxy(partial(_lookup_req_object, 'service'))
 # The current organisation attached to the request stack.
 current_organisation = LocalProxy(partial(_lookup_req_object, 'organisation'))
 
-main_navigation = MainNavigation()
+navigation = {
+    'main_navigation': MainNavigation(),
+    'header_navigation': HeaderNavigation(),
+}
 
 
 def create_app(application):
@@ -176,7 +179,7 @@ def init_app(application):
 
     @application.context_processor
     def _nav_selected():
-        return {'main_navigation': main_navigation}
+        return navigation
 
     @application.before_request
     def record_start_time():