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/postgres/000042_create_threads.up.sql

@@ -0,0 +1,35 @@
+CREATE TABLE IF NOT EXISTS threads (
+    postid VARCHAR(26) PRIMARY KEY,
+    replycount bigint,
+    lastreplyat bigint,
+    participants text
+);
+
+ALTER TABLE threads ADD COLUMN IF NOT EXISTS channelid VARCHAR(26);
+
+CREATE INDEX IF NOT EXISTS idx_threads_channel_id ON threads (channelid);
+
+DO $$
+	<< migrate_empty_threads >>
+DECLARE
+	empty_threads_exist boolean := FALSE;
+BEGIN
+	SELECT
+		count(*) != 0 INTO empty_threads_exist
+	FROM
+		threads
+	WHERE
+		channelid IS NULL;
+	IF empty_threads_exist THEN
+		UPDATE
+			threads
+		SET
+			channelId = posts.channelid
+		FROM
+			posts
+		WHERE
+			posts.id = threads.postid
+			AND threads.channelid IS NULL;
+	END IF;
+END migrate_empty_threads
+$$;