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.down.sql

@@ -0,0 +1,5 @@
+DROP INDEX IF EXISTS idx_threads_channel_id;
+
+ALTER TABLE threads DROP COLUMN IF EXISTS channelid;
+
+DROP TABLE IF EXISTS threads;