[ticket/17525] Avoid more index name duplication

Many more tables have indexes with the same names which can cause issues
on SQLite/Postgres. Also, add an option for migrations to rename indexes.

PHPBB-17525

phpBB/phpbb/db/migration/helper.php

@@ -42,6 +42,7 @@ class helper
 			'add_primary_keys'	=> 2, // perform_schema_changes only uses one level, but second is in the function
 			'add_unique_index'	=> 2,
 			'add_index'			=> 2,
+			'rename_index'		=> 1,
 		);
 
 		foreach ($nested_level as $change_type => $data_depth)