fix: add render_grid_tag() as separate def block for index templates

sometimes caller needs to inject custom attrs to the grid tag

src/wuttaweb/templates/master/index.mako

@@ -8,10 +8,14 @@
 
 <%def name="page_content()">
   % if grid is not Undefined:
-      ${grid.render_vue_tag()}
+      ${self.render_grid_tag()}
   % endif
 </%def>
 
+<%def name="render_grid_tag()">
+  ${grid.render_vue_tag()}
+</%def>
+
 <%def name="render_vue_templates()">
   ${parent.render_vue_templates()}
   ${self.render_vue_template_grid()}