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/constants.go

@@ -0,0 +1,8 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package app
+
+const MissingChannelMemberError = "app.channel.get_member.missing.app_error"
+const MissingAccountError = "app.user.missing_account.const"
+const MissingAuthAccountError = "app.user.get_by_auth.missing_account.app_error"