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/000020_create_posts.down.sql

@@ -0,0 +1,3 @@
+CREATE INDEX IF NOT EXISTS idx_posts_channel_id ON posts (channelid);
+
+DROP TABLE IF EXISTS posts;