Add default page size for reports

- A4 / Legal / Letter
- Use this for the default page size for InvenTree reports

InvenTree/templates/InvenTree/settings/report.html

@@ -15,6 +15,7 @@
 <table class='table table-striped table-condensed'>
     {% include "InvenTree/settings/header.html" %}
     <tbody>
+        {% include "InvenTree/settings/setting.html" with key="REPORT_DEFAULT_PAGE_SIZE" %}
         {% include "InvenTree/settings/setting.html" with key="REPORT_ENABLE_TEST_REPORT" %}
     </tbody>
 </table>