[FIX] website_slides: add missing class to show discussion tab panel

Task #1923443

Purpose
=======

Probably due to the migration to bootstrap 4, the active tab panel in the 'slide_detail_view' was not displayed
on page loading (and only when switching back and forth in the tabs).

partly closes #29901

addons/website_slides/views/website_slides.xml

@@ -508,7 +508,7 @@
                                 </li>
                             </ul>
                             <div class="tab-content" style="padding: 20px 5px 5px 5px; word-wrap: break-word;">
-                                <div role="tabpanel" t-att-class="not comments and 'tab-pane fade active in' or 'tab-pane fade'" id="about">
+                                <div role="tabpanel" t-att-class="not comments and 'tab-pane fade in show active' or 'tab-pane fade'" id="about">
                                     <div t-field="slide.description"/>
                                 </div>
                                 <div role="tabpanel" class="tab-pane fade" t-if="slide.website_published" id="share">
@@ -529,7 +529,7 @@
                                         The social sharing module will be unlocked when a moderator will allow your publication.
                                     </h4>
                                 </div>
-                                <div role="tabpanel" t-att-class="comments and 'tab-pane fade active in' or 'tab-pane fade'" id="discuss">
+                                <div role="tabpanel" t-att-class="comments and 'tab-pane fade in show active' or 'tab-pane fade'" id="discuss">
                                     <t t-call="portal.message_thread">
                                         <t t-set="object" t-value="slide"/>
                                     </t>