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/000042_create_threads.up.sql

@@ -0,0 +1,60 @@
+CREATE TABLE IF NOT EXISTS Threads (
+    PostId varchar(26) NOT NULL,
+    ReplyCount bigint(20),
+    LastReplyAt bigint(20),
+    Participants text,
+    PRIMARY KEY (PostId)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+SET @preparedStatement = (SELECT IF(
+    (
+        SELECT COUNT(*) FROM INFORMATION_SCHEMA.COLUMNS
+        WHERE table_name = 'Threads'
+        AND table_schema = DATABASE()
+        AND column_name = 'ChannelId'
+    ) > 0,
+    'SELECT 1',
+    'ALTER TABLE Threads ADD COLUMN ChannelId VARCHAR(26) DEFAULT NULL;'
+));
+
+PREPARE alterIfNotExists FROM @preparedStatement;
+EXECUTE alterIfNotExists;
+DEALLOCATE PREPARE alterIfNotExists;
+
+SET @preparedStatement = (SELECT IF(
+    (
+        SELECT COUNT(*) FROM INFORMATION_SCHEMA.STATISTICS
+        WHERE table_name = 'Threads'
+        AND table_schema = DATABASE()
+        AND index_name = 'idx_threads_channel_id'
+    ) > 0,
+    'SELECT 1',
+    'CREATE INDEX idx_threads_channel_id ON Threads(ChannelId);'
+));
+
+PREPARE createIndexIfNotExists FROM @preparedStatement;
+EXECUTE createIndexIfNotExists;
+DEALLOCATE PREPARE createIndexIfNotExists;
+
+CREATE PROCEDURE Migrate_Empty_Threads ()
+BEGIN
+DECLARE
+	EMPTY_THREADS_EXIST INT;
+	SELECT
+		COUNT(*)
+	FROM
+		Threads
+	WHERE
+		ChannelId IS NULL INTO EMPTY_THREADS_EXIST;
+	IF(EMPTY_THREADS_EXIST > 0) THEN
+		UPDATE
+			Threads
+			INNER JOIN Posts ON Posts.Id = Threads.PostId
+		SET
+			Threads.ChannelId = Posts.ChannelId
+		WHERE
+			Threads.ChannelId IS NULL;
+		END IF;
+END;
+	CALL Migrate_Empty_Threads ();
+	DROP PROCEDURE IF EXISTS Migrate_Empty_Threads;