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

@@ -0,0 +1,20 @@
+{% extends "views/platform-admin/_base_template.html" %}
+{% from "components/form.html" import form_wrapper %}
+
+{% block per_page_title %}
+  Billing Report
+{% endblock %}
+
+{% block platform_admin_content %}
+
+  <h1 class="heading-large">
+    Billing Report
+  </h1>
+
+  {% call form_wrapper() %}
+    {{ form.start_date(param_extensions={"hint": {"text": "Enter start date in format YYYY-MM-DD"}}) }}
+    {{ form.end_date(param_extensions={"hint": {"text": "Enter end date in format YYYY-MM-DD"}}) }}
+    {{ page_footer('Download report') }}
+  {% endcall %}
+
+{% endblock %}