[FIX] website_forum: add missing ID which lead to broken JS

That ID was removed with c8c8eb3d, but the JS code is expecting to find
this ID inside the DOM to increment the flag counter.

Courtesy of @dwa-odoo
Spotted while working on task-2167561

closes odoo/odoo#74796

Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>

addons/website_forum/views/website_forum.xml

@@ -561,7 +561,7 @@
             </span>
             <a t-else="" class="nav-link rounded-pill text-reset" t-attf-href="/forum/#{slug(forum)}/flagged_queue">
                 <i class="fa fa-flag fa-fw"/> Flagged
-                <span t-attf-class="badge pull-right #{forum.count_flagged_posts > 0 and 'badge-danger' or 'badge-light'}" t-esc="forum.count_flagged_posts"/>
+                <span id="count_flagged_posts" t-attf-class="badge pull-right #{forum.count_flagged_posts > 0 and 'badge-danger' or 'badge-light'}" t-esc="forum.count_flagged_posts"/>
             </a>
         </t>
     </nav>