Mono repo -> Master (#22553)

Combines the following repositories into one:

https://github.com/mattermost/mattermost-server
https://github.com/mattermost/mattermost-webapp
https://github.com/mattermost/focalboard
https://github.com/mattermost/mattermost-plugin-playbooks

server/channels/app/cluster.go

@@ -0,0 +1,20 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package app
+
+// Registers a given function to be called when the cluster leader may have changed. Returns a unique ID for the
+// listener which can later be used to remove it. If clustering is not enabled in this build, the callback will never
+// be called.
+func (s *Server) AddClusterLeaderChangedListener(listener func()) string {
+	return s.platform.AddClusterLeaderChangedListener(listener)
+}
+
+// Removes a listener function by the unique ID returned when AddConfigListener was called
+func (s *Server) RemoveClusterLeaderChangedListener(id string) {
+	s.platform.RemoveClusterLeaderChangedListener(id)
+}
+
+func (s *Server) InvokeClusterLeaderChangedListeners() {
+	s.platform.InvokeClusterLeaderChangedListeners()
+}