[FIX] website_event_booth: allow the user to drop snippets

This commit allows the user to drop snippets before and after the
booth's selection page content.

task-3718417
part of task-3698364

Part-of: odoo/odoo#152434

addons/website_event_booth/views/event_booth_templates.xml

@@ -3,6 +3,7 @@
 
     <template id="event_booth_layout" name="Event Booth Layout">
         <t t-call="website_event.layout">
+            <div class="oe_structure oe_empty" id="oe_structure_website_event_booth_1"/>
             <div class="o_wbooth_registration" t-att-data-event-id="event.id"
                  itemscope="itemscope" itemtype="http://schema.org/Event">
                 <section>
@@ -38,6 +39,7 @@
                     </div>
                 </div>
             </div>
+             <div class="oe_structure oe_empty" id="oe_structure_website_event_booth_2"/>
         </t>
     </template>