Stop using platform_admin_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

For most tests of a platform admin view we used `platform_admin_client`
instead. This is not as good because it returns a raw `Response` object
and doesn’t do the additional checks.

This commit converts all the tests using `platform_admin_client` to:
use new `client_request` and log in as `platform_admin_user` before
making any requests.

This is also nice because it makes any test easy to parametrize with
additional users, for example to test differences in behaviour dependant
on being platform admin or not.

app/templates/views/platform-admin/complaints.html

@@ -4,7 +4,7 @@
 {% from "components/table.html" import list_table, field, text_field, link_field, right_aligned_field_heading, hidden_field_heading %}
 
 {% block per_page_title %}
-  {{ page_title|capitalize }}
+  Email complaints
 {% endblock %}
 
 {% block platform_admin_content %}

app/templates/views/platform-admin/index.html

@@ -6,7 +6,7 @@
 {% from "components/button/macro.njk" import govukButton %}
 
 {% block per_page_title %}
-  Platform admin
+  Summary
 {% endblock %}
 
 {% block platform_admin_content %}

app/templates/views/platform-admin/reports.html

@@ -1,7 +1,7 @@
 {% extends "views/platform-admin/_base_template.html" %}
 
 {% block per_page_title %}
-  Platform admin
+  Reports
 {% endblock %}
 
 {% block platform_admin_content %}