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/db/migrations/mysql/000035_create_sharedchannelattachments.up.sql

@@ -0,0 +1,9 @@
+CREATE TABLE IF NOT EXISTS SharedChannelAttachments (
+  Id varchar(26) NOT NULL,
+  FileId varchar(26) DEFAULT NULL,
+  RemoteId varchar(26) DEFAULT NULL,
+  CreateAt bigint(20) DEFAULT NULL,
+  LastSyncAt bigint(20) DEFAULT NULL,
+  PRIMARY KEY (Id),
+  UNIQUE KEY FileId (FileId,RemoteId)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;