Replace uses of client.get and client.post

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 `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 which had a `client.get(…)` or
`client.post(…)` statement to use their equivalents on `client_request`
instead.

Subsequent commits will remove uses of `client` in other tests, but
doing it this way means the work can be broken up into more manageable
chunks.

tests/conftest.py

@@ -2904,9 +2904,17 @@ def client_request(_logged_in_client, mocker, service_one):  # noqa (C901 too co
             _optional_args="",
             **endpoint_kwargs
         ):
-            resp = _logged_in_client.get(
+            return ClientRequest.get_response_from_url(
                 url_for(endpoint, **(endpoint_kwargs or {})) + _optional_args,
+                _expected_status=_expected_status,
             )
+
+        @staticmethod
+        def get_response_from_url(
+            url,
+            _expected_status=200,
+        ):
+            resp = _logged_in_client.get(url)
             assert resp.status_code == _expected_status
             return resp