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/000015_create_systems.up.sql

@@ -0,0 +1,5 @@
+CREATE TABLE IF NOT EXISTS systems (
+    name VARCHAR(64),
+    value VARCHAR(1024),
+    PRIMARY KEY (name)
+);