fix: add render_form_tag() customization hook in /form template

src/wuttaweb/templates/form.mako

@@ -17,11 +17,15 @@
 <%def name="page_content()">
   % if form is not Undefined:
       <div class="wutta-form-wrapper">
-        ${form.render_vue_tag()}
+        ${self.render_form_tag()}
       </div>
   % endif
 </%def>
 
+<%def name="render_form_tag()">
+  ${form.render_vue_tag()}
+</%def>
+
 <%def name="tool_panels_wrapper()">
   <div class="tool-panels-wrapper">
     ${self.tool_panels()}