Stop using logged_in_client fixture

We have a `client_request` fixture which does a bunch of useful stuff
like:
- checking the status code of the response
- returning a `BeautifulSoup` object

Lots of our tests still use an older fixture called `logged_in_client`.
This is not as good because:
- it returns a raw `Response` object
- doesn’t do the additional checks
- means our tests contain a lot of repetetive boilerplate like `page = BeautifulSoup(response.data.decode('utf-8'), 'html.parser')`

This commit converts all the tests using `logged_in_client` to:
use `client_request` instead.

tests/app/main/views/test_broadcast.py

@@ -555,17 +555,15 @@ def test_broadcast_dashboard_has_new_alert_button_if_user_has_permission_to_crea
 
 @freeze_time('2020-02-20 02:20')
 def test_broadcast_dashboard_json(
-    logged_in_client,
+    client_request,
     service_one,
     mock_get_broadcast_messages,
 ):
     service_one['permissions'] += ['broadcast']
-    response = logged_in_client.get(url_for(
+    response = client_request.get_response(
         '.broadcast_dashboard_updates',
         service_id=SERVICE_ONE_ID,
-    ))
-
-    assert response.status_code == 200
+    )
 
     json_response = json.loads(response.get_data(as_text=True))