Add 'REPORT_DEBUG_MODE' setting

- If set, reports are printed in raw HTML
- Not pretty, but useful for debugging output of rendered template

InvenTree/templates/InvenTree/settings/report.html

@@ -16,6 +16,7 @@
     {% 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_DEBUG_MODE" %}
         {% include "InvenTree/settings/setting.html" with key="REPORT_ENABLE_TEST_REPORT" %}
     </tbody>
 </table>