Simplify the new template rendering

- No extra template code is required for any new page
- All loaded in base.html or page_base.html
- Oh, so clean!

InvenTree/templates/page_base.html

@@ -61,6 +61,8 @@
 </div>
 
 {% block page_content %}
+<!-- Custom page content goes here-->
 {% endblock %}
+{% include "plugin/panel_content.html" %}
 
 {% endblock %}