[FIX] mail: restore background color of notes in chatter

It had been removed by the new design of v11, but it is necessary
to easily make the distinction between internal notes and messages.

addons/mail/static/src/less/thread.less

@@ -104,6 +104,10 @@
             max-width: 100%;
             word-wrap: break-word;
 
+            &.o_mail_note {
+                background-color: @odoo-color-silver-dark;
+            }
+
             .o_mail_subject {
                 font-style: italic;
             }