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/000036_create_sharedchannelusers.up.sql

@@ -0,0 +1,26 @@
+CREATE TABLE IF NOT EXISTS sharedchannelusers (
+    id varchar(26) NOT NULL,
+    userid varchar(26),
+    remoteid varchar(26),
+    createat bigint,
+    lastsyncat bigint,
+    PRIMARY KEY (id)
+);
+
+ALTER TABLE sharedchannelusers ADD COLUMN IF NOT EXISTS channelid varchar(26);
+
+DO $$
+BEGIN
+    IF NOT EXISTS (
+       SELECT conname
+       FROM pg_constraint
+       WHERE conname = 'sharedchannelusers_userid_channelid_remoteid_key'
+    )
+    THEN
+        ALTER TABLE sharedchannelusers ADD CONSTRAINT sharedchannelusers_userid_channelid_remoteid_key UNIQUE (userid, channelid, remoteid);
+    END IF;
+END $$;
+
+DROP INDEX IF EXISTS idx_sharedchannelusers_user_id;
+
+CREATE INDEX IF NOT EXISTS idx_sharedchannelusers_remote_id ON sharedchannelusers(remoteid);