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/main/views/platform_admin.py

@@ -274,7 +274,7 @@ def notifications_sent_by_service():
 
 @main.route("/platform-admin/reports/usage-for-all-services", methods=['GET', 'POST'])
 @user_is_platform_admin
-def usage_for_all_services():
+def get_billing_report():
     form = RequiredDateFilterForm()
 
     if form.validate_on_submit():
@@ -286,7 +286,7 @@ def usage_for_all_services():
             "contact_names", "contact_email_addresses", "billing_reference"
         ]
 
-        result = billing_api_client.get_usage_for_all_services(start_date, end_date)
+        result = billing_api_client.get_data_for_billing_report(start_date, end_date)
         rows = [
             [
                 r["organisation_id"], r["organisation_name"], r["service_id"], r["service_name"],
@@ -299,13 +299,13 @@ def usage_for_all_services():
         if rows:
             return Spreadsheet.from_rows([headers] + rows).as_csv_data, 200, {
                 'Content-Type': 'text/csv; charset=utf-8',
-                'Content-Disposition': 'attachment; filename="Usage for all services from {} to {}.csv"'.format(
+                'Content-Disposition': 'attachment; filename="Billing Report from {} to {}.csv"'.format(
                     start_date, end_date
                 )
             }
         else:
             flash('No results for dates')
-    return render_template('views/platform-admin/usage_for_all_services.html', form=form)
+    return render_template('views/platform-admin/get-billing-report.html', form=form)
 
 
 @main.route("/platform-admin/complaints")