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/000043_thread_memberships.down.sql

@@ -0,0 +1,7 @@
+DROP INDEX IF EXISTS idx_thread_memberships_user_id;
+DROP INDEX IF EXISTS idx_thread_memberships_last_view_at;
+DROP INDEX IF EXISTS idx_thread_memberships_last_update_at;
+
+ALTER TABLE threadmemberships DROP COLUMN IF EXISTS unreadmentions;
+
+DROP TABLE IF EXISTS theadmemberships;