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/000050_create_channelmembers.down.sql

@@ -0,0 +1,10 @@
+CREATE INDEX IF NOT EXISTS idx_channelmembers_channel_id ON channelmembers(channelid);
+
+ALTER TABLE channelmembers DROP COLUMN IF EXISTS schemeguest;
+
+ALTER TABLE channelmembers DROP COLUMN IF EXISTS schemeadmin;
+ALTER TABLE channelmembers DROP COLUMN IF EXISTS schemeuser;
+
+DROP INDEX IF EXISTS idx_channelmembers_user_id;
+
+DROP TABLE IF EXISTS channelmembers;