Make the broadcast dashboard update via AJAX

Same technique as we use for other pages that update via AJAX.

I’ve split the page up into separate chunks because the DiffDOM library
we use finds it easier to work out what’s changed when there are fewer
elements/a shallower tree.

app/navigation.py

@@ -353,6 +353,7 @@ class HeaderNavigation(Navigation):
         'who_can_use_notify',
         'who_its_for',
         'broadcast_dashboard',
+        'broadcast_dashboard_updates',
         'broadcast',
         'preview_broadcast_areas',
         'choose_broadcast_library',
@@ -373,6 +374,7 @@ class MainNavigation(Navigation):
     mapping = {
         'dashboard': {
             'broadcast_dashboard',
+            'broadcast_dashboard_updates',
             'conversation',
             'inbox',
             'monthly',
@@ -697,6 +699,7 @@ class CaseworkNavigation(Navigation):
     mapping = {
         'dashboard': {
             'broadcast_dashboard',
+            'broadcast_dashboard_updates',
         },
         'send-one-off': {
             'choose_from_contact_list',
@@ -1322,6 +1325,7 @@ class OrgNavigation(Navigation):
         'who_can_use_notify',
         'who_its_for',
         'broadcast_dashboard',
+        'broadcast_dashboard_updates',
         'broadcast',
         'preview_broadcast_areas',
         'choose_broadcast_library',