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/000083_threads_threaddeleteat.up.sql

@@ -0,0 +1,35 @@
+-- Drop any existing DeleteAt column from 000081_threads_deleteat.up.sql
+SET @preparedStatement = (SELECT IF(
+    EXISTS(
+        SELECT 1 FROM INFORMATION_SCHEMA.STATISTICS
+        WHERE table_name = 'Threads'
+        AND table_schema = DATABASE()
+        AND column_name = 'DeleteAt'
+    ) > 0,
+    'ALTER TABLE Threads DROP COLUMN DeleteAt;',
+    'SELECT 1;'
+));
+
+PREPARE removeColumnIfExists FROM @preparedStatement;
+EXECUTE removeColumnIfExists;
+DEALLOCATE PREPARE removeColumnIfExists;
+
+SET @preparedStatement = (SELECT IF(
+    NOT EXISTS(
+        SELECT 1 FROM INFORMATION_SCHEMA.COLUMNS
+        WHERE table_name = 'Threads'
+        AND table_schema = DATABASE()
+        AND column_name = 'ThreadDeleteAt'
+    ),
+    'ALTER TABLE Threads ADD COLUMN ThreadDeleteAt bigint(20);',
+    'SELECT 1;'
+));
+
+PREPARE addColumnIfNotExists FROM @preparedStatement;
+EXECUTE addColumnIfNotExists;
+DEALLOCATE PREPARE addColumnIfNotExists;
+
+UPDATE Threads, Posts
+SET Threads.ThreadDeleteAt = Posts.DeleteAt
+WHERE Posts.Id = Threads.PostId
+AND Threads.ThreadDeleteAt IS NULL;