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.

tests/app/main/views/test_broadcast.py

@@ -1,3 +1,5 @@
+import json
+
 import pytest
 from flask import url_for
 from freezegun import freeze_time
@@ -10,6 +12,7 @@ sample_uuid = sample_uuid()
 
 @pytest.mark.parametrize('endpoint, extra_args', (
     ('.broadcast_dashboard', {}),
+    ('.broadcast_dashboard_updates', {}),
     ('.broadcast', {'template_id': sample_uuid}),
     ('.preview_broadcast_areas', {'broadcast_message_id': sample_uuid}),
     ('.choose_broadcast_library', {'broadcast_message_id': sample_uuid}),
@@ -88,6 +91,28 @@ def test_broadcast_dashboard(
     ]
 
 
+@freeze_time('2020-02-20 02:20')
+def test_broadcast_dashboard_json(
+    logged_in_client,
+    service_one,
+    mock_get_broadcast_messages,
+):
+    service_one['permissions'] += ['broadcast']
+    response = logged_in_client.get(url_for(
+        '.broadcast_dashboard_updates',
+        service_id=SERVICE_ONE_ID,
+    ))
+
+    assert response.status_code == 200
+
+    json_response = json.loads(response.get_data(as_text=True))
+
+    assert json_response.keys() == {'live_broadcasts', 'previous_broadcasts'}
+
+    assert 'Live until tomorrow at 2:20am' in json_response['live_broadcasts']
+    assert 'Finished yesterday at 8:20pm' in json_response['previous_broadcasts']
+
+
 def test_broadcast_page(
     client_request,
     service_one,