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/000042_create_threads.down.sql

@@ -0,0 +1,31 @@
+SET @preparedStatement = (SELECT IF(
+    (
+        SELECT COUNT(*) FROM INFORMATION_SCHEMA.STATISTICS
+        WHERE table_name = 'Threads'
+        AND table_schema = DATABASE()
+        AND index_name = 'idx_threads_channel_id'
+    ) > 0,
+    'DROP INDEX idx_threads_channel_id ON Threads;',
+    'SELECT 1'
+));
+
+PREPARE removeIndexIfExists FROM @preparedStatement;
+EXECUTE removeIndexIfExists;
+DEALLOCATE PREPARE removeIndexIfExists;
+
+SET @preparedStatement = (SELECT IF(
+    (
+        SELECT COUNT(*) FROM INFORMATION_SCHEMA.COLUMNS
+        WHERE table_name = 'Threads'
+        AND table_schema = DATABASE()
+        AND column_name = 'ChannelId'
+    ) > 0,
+    'ALTER TABLE Threads DROP COLUMN ChannelId;',
+    'SELECT 1'
+));
+
+PREPARE alterIfExists FROM @preparedStatement;
+EXECUTE alterIfExists;
+DEALLOCATE PREPARE alterIfExists;
+
+DROP TABLE IF EXISTS Threads;