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/copyright.txt

@@ -0,0 +1,2 @@
+Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+See LICENSE.txt for license information.