[FIX] mail: content override toolbar in mobile landscape

On mobile (specifically in landscape orientation as the height of the
viewport is narrower), the content appear over the bottom toolbar
instead of going below.
Solution is in 2 parts:
* raise the toolbar way above the content
* make the toolbar opaque

closes odoo/odoo#28429

addons/mail/static/src/scss/discuss.scss

@@ -279,6 +279,8 @@
         }
         .o_mail_mobile_tabs {
             flex: 0 0 auto;
+            z-index: 1;
+            background-color: white;
         }
     }
 }