Make existing tests use ``client_request fixture

This commit makes the existing tests around user permissions less
verbose by using the new `client_request` fixture.

This fixture takes care of:
- setting up a service
- asserting that the response is `200`

It also tests that the page titles, some of which didn’t match with the
`<h1>`s, so this commit also fixes that mismatch.

app/templates/views/edit-user-permissions.html

@@ -3,7 +3,7 @@
 {% from "components/page-footer.html" import page_footer %}
 
 {% block service_page_title %}
-Manage users
+  {{ user.name or user.email_localpart }}
 {% endblock %}
 
 {% block maincolumn_content %}

app/templates/views/manage-users.html

@@ -12,7 +12,7 @@
 } %}
 
 {% block service_page_title %}
-Manage users
+  Team members
 {% endblock %}
 
 {% block maincolumn_content %}