[FIX] website_forum: allow themes to customize navbar area

Before the forum redesign, the .container element which contains the
forum navbar had the 'website_forum' class. Now this class was moved
to encapsulate the whole forum page.

Problem is that themes were styling that container element specifically
and the `#wrap` element of the forum by respectively targeting
`.website_forum` and `.website_forum + #wrap`. This commit solves this
by adding a 'o_wforum_navbar_container' class on the element allowing
to modify the themes selector with `.o_wforum_navbar_container` and
`.website_forum #wrap`.

closes odoo/odoo#29581

addons/website_forum/views/website_forum.xml

@@ -52,7 +52,7 @@
             <div t-field="forum.welcome_message"/>
         </div>
 
-        <div class="container mt16 mb32">
+        <div class="container mt16 mb32 o_wforum_navbar_container">
             <div class="navbar navbar-expand-md navbar-light">
                 <a class="navbar-brand" t-attf-href="/forum/#{slug(forum)}">
                     <span t-field="forum.name"/>