[FIX] web: KanbanView, layout issues

Prior to this commit there were some layout issues in KanbanView:

1. The `o_kanban_header` z-index was not high enough and some icons in
the record element could hide it partially.

2. The `o_ghost_avatar` was not visible (white on white).

3. `.container` elements placed inside a record had their width
   constrained by the grid system. This commit "fix the issue" setting
   their max-width to 'none'. 

Note on point 3:
To have '.container' max-width limited by the current resolution is not
a bug but the default grid-system behavior. This commit provides a
temporary workaround since 'container-fluid' classes should have been
used instead.

task-2892062

Part-of: odoo/odoo#96195

addons/web/static/src/legacy/scss/kanban_dashboard.scss

@@ -19,6 +19,7 @@
         // ------- Generic layout adaptations -------
         .container {
             width: 100%;
+            max-width: none;
         }
 
         // ------- Dropdown toggle & menu -------

addons/web/static/src/views/kanban/kanban_dashboard.scss

@@ -13,6 +13,7 @@
         // ------- Generic layout adaptations -------
         .container {
             width: 100%;
+            max-width: none;
         }
 
         // ------- Dropdown toggle & menu -------

addons/web/static/src/views/kanban/kanban_renderer.xml

@@ -153,8 +153,9 @@
                                 <div t-foreach="column.cards" t-as="card" t-key="card_index" class="o_kanban_examples_ghost d-flex flex-wrap justify-content-between mb-3 p-2 border bg-white">
                                     <div class="o_ghost_content w-100 pb-3 bg-300"/>
                                     <div class="o_ghost_content o_ghost_tag d-inline-block w-50 mt-3 pb-3 bg-300"/>
-                                    <!-- FIXME WOWL example avatar is white on white (also in legacy) -->
-                                    <img class="o_ghost_avatar" src="/base/static/img/avatar.png" alt="Avatar"/>
+                                    <span class="mt-2 rounded-circle bg-300">
+                                        <img class="o_ghost_avatar" src="/base/static/img/avatar.png" alt="Avatar"/>
+                                    </span>
                                 </div>
                             </div>
                         </div>