Go to broadcast, not dashboard after submitting

Once you’ve created a broadcast you’re taken back to the dashboard. This
feels too passive, and you might miss that the broadcast still needs
approval.

We should be much more explicit that you now need to find someone to
approve your broadcast. Taking someone directly to the page for a
broadcast lets us give more information about the status of the
broadcast and what the next steps should be.

app/main/views/broadcast.py

@@ -169,8 +169,9 @@ def preview_broadcast_message(service_id, broadcast_message_id):
     if form.validate_on_submit():
         broadcast_message.request_approval(until=form.finishes_at.data)
         return redirect(url_for(
-            '.broadcast_dashboard',
+            '.view_broadcast_message',
             service_id=current_service.id,
+            broadcast_message_id=broadcast_message.id,
         ))
 
     return render_template(

tests/app/main/views/test_broadcast.py

@@ -412,7 +412,13 @@ def test_start_broadcasting(
         broadcast_message_id=fake_uuid,
         _data={
             'finishes_at': end_time,
-        }
+        },
+        _expected_redirect=url_for(
+            'main.view_broadcast_message',
+            service_id=SERVICE_ONE_ID,
+            broadcast_message_id=fake_uuid,
+            _external=True,
+        ),
     ),
     mock_update_broadcast_message.assert_called_once_with(
         service_id=SERVICE_ONE_ID,