[IMP] mail: sidebar autocomplete: redirect to already opened DM

Before this rev., if the user clicked, in the autocomplete dropdown, on an
already opened DM, nothing happened. Now, it redirects to that DM.

Also clear the input once a partner is selected.

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

@@ -339,7 +339,15 @@ var ChatAction = Widget.extend(ControlPanelMixin, {
             },
             select: function(event, ui) {
                 var partner_id = ui.item.id;
-                chat_manager.create_channel(partner_id, "dm");
+                var dm = chat_manager.get_dm_from_partner_id(partner_id);
+                if (dm) {
+                    self.set_channel(dm);
+                } else {
+                    chat_manager.create_channel(partner_id, "dm");
+                }
+                // clear the input
+                $(this).val('');
+                return false;
             },
             focus: function(event) {
                 event.preventDefault();