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/mysql/000020_create_posts.down.sql

@@ -0,0 +1,16 @@
+SET @preparedStatement = (SELECT IF(
+    (
+        SELECT COUNT(*) FROM INFORMATION_SCHEMA.STATISTICS
+        WHERE table_name = 'Posts'
+        AND table_schema = DATABASE()
+        AND index_name = 'idx_posts_channel_id'
+    ) > 0,
+    'SELECT 1',
+    'CREATE INDEX idx_posts_channel_id ON Posts(ChannelId);'
+));
+
+PREPARE createIndexIfNotExists FROM @preparedStatement;
+EXECUTE createIndexIfNotExists;
+DEALLOCATE PREPARE createIndexIfNotExists;
+
+DROP TABLE IF EXISTS Posts;