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.

app/templates/views/signin.html

@@ -3,7 +3,11 @@
 {% from "components/form.html" import form_wrapper %}
 
 {% block per_page_title %}
-  Sign in
+  {% if again %}
+    You need to sign in again
+  {% else %}
+    Sign in
+  {% endif %}
 {% endblock %}
 
 {% block maincolumn_content %}