Rename usage report to billing report

Because only services with bills to pay are included, and
we started including billing details.

Also rename endpoints and file names to match this.

app/templates/views/platform-admin/get-billing-report.html

@@ -2,13 +2,13 @@
 {% from "components/form.html" import form_wrapper %}
 
 {% block per_page_title %}
-  Usage for all services
+  Billing Report
 {% endblock %}
 
 {% block platform_admin_content %}
 
   <h1 class="heading-large">
-    Usage for all services
+    Billing Report
   </h1>
 
   {% call form_wrapper() %}

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

@@ -17,6 +17,6 @@
   <a class="govuk-link govuk-link--no-visited-state" href="{{ url_for('main.notifications_sent_by_service') }}">Monthly notification statuses for live services</a>
 </p>
 <p class="govuk-body">
-  <a class="govuk-link govuk-link--no-visited-state" href="{{ url_for('main.usage_for_all_services') }}">Usage for all services</a>
+  <a class="govuk-link govuk-link--no-visited-state" href="{{ url_for('main.get_billing_report') }}">Billing Report</a>
 </p>
 {% endblock %}