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/000076_upgrade_lastrootpostat.up.sql

@@ -0,0 +1,47 @@
+CREATE PROCEDURE Migrate_LastRootPostAt_Default ()
+BEGIN
+	IF (
+			SELECT COUNT(*) FROM INFORMATION_SCHEMA.COLUMNS
+			WHERE TABLE_NAME = 'Channels'
+			AND TABLE_SCHEMA = DATABASE()
+			AND COLUMN_NAME = 'LastRootPostAt'
+			AND (COLUMN_DEFAULT IS NULL OR COLUMN_DEFAULT != 0)
+		) = 1 THEN
+		ALTER TABLE Channels ALTER COLUMN LastRootPostAt SET DEFAULT 0;
+	END IF;
+END;
+	CALL Migrate_LastRootPostAt_Default ();
+	DROP PROCEDURE IF EXISTS Migrate_LastRootPostAt_Default;
+
+CREATE PROCEDURE Migrate_LastRootPostAt_Fix ()
+BEGIN
+	IF (
+		SELECT COUNT(*)
+		FROM Channels
+		WHERE LastRootPostAt IS NULL
+	) > 0 THEN
+	-- fixes migrate cte and sets the LastRootPostAt for channels that don't have it set
+		UPDATE
+			Channels
+			INNER JOIN (
+				SELECT
+					Channels.Id channelid,
+					COALESCE(MAX(Posts.CreateAt), 0) AS lastrootpost
+				FROM
+					Channels
+					LEFT JOIN Posts FORCE INDEX (idx_posts_channel_id_update_at) ON Channels.Id = Posts.ChannelId
+				WHERE
+					Posts.RootId = ''
+				GROUP BY
+					Channels.Id) AS q ON q.channelid = Channels.Id
+				SET
+					LastRootPostAt = lastrootpost
+				WHERE
+					LastRootPostAt IS NULL;
+
+		-- sets LastRootPostAt to 0, for channels with no posts
+		UPDATE Channels SET LastRootPostAt=0 WHERE LastRootPostAt IS NULL;
+	END IF;
+END;
+	CALL Migrate_LastRootPostAt_Fix ();
+	DROP PROCEDURE IF EXISTS Migrate_LastRootPostAt_Fix;