Rewrite navigation as a class

Because we have multiple navigations, which will share the same methods
(by subclassing) but different mappings of navigation items to endpoints
by overriding the `.mapping` and `.exclude` attributes.

app/__init__.py

@@ -36,9 +36,10 @@ from notifications_utils.formatters import formatted_list
 from werkzeug.exceptions import abort
 from werkzeug.local import LocalProxy
 
-from app import proxy_fix, navigation
+from app import proxy_fix
 from app.config import configs
 from app.asset_fingerprinter import AssetFingerprinter
+from app.navigation import 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
@@ -89,6 +90,8 @@ 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()
+
 
 def create_app(application):
     setup_commands(application)
@@ -173,7 +176,7 @@ def init_app(application):
 
     @application.context_processor
     def _nav_selected():
-        return {'nav_selected': navigation.nav_selected}
+        return {'main_navigation': main_navigation}
 
     @application.before_request
     def record_start_time():