[FIX] mail: Discuss: channel autocomplete dropdown

The content of this dropdown must be html (for the 'Create' line). I
accidentally removed it at 33beee5.

addons/mail/static/src/js/client_action.js

@@ -330,6 +330,7 @@ var ChatAction = Widget.extend(ControlPanelMixin, {
             focus: function(event) {
                 event.preventDefault();
             },
+            html: true,
         });
 
         this.$('.o_mail_add_channel[data-type=dm]').find("input").autocomplete({