When running MySQL/MariaDB with charset utf8mb4, a character is 4 bytes. Hence a key created on a VARCHAR(255) takes 255 characters * 4 bytes/characters = 1020 bytes. However Innodb defaults to a maximum of 767 bytes for an index.
The extension fails to install on Debian Stretch which uses utf8mb4 as the default charset:
```
Query: CREATE TABLE IF NOT EXISTS `flaggedtemplates` (
ft_rev_id integer unsigned NOT NULL,
ft_namespace int NOT NULL default '0',
ft_title varchar(255) binary NOT NULL default '',
ft_tmp_rev_id integer unsigned NULL,
PRIMARY KEY (ft_rev_id,ft_namespace,ft_title)
) ENGINE=InnoDB
Function: Wikimedia\Rdbms\Database::sourceFile( /workspace/src/extensions/FlaggedRevs/backend/schema/mysql/FlaggedRevs.sql )
Error: 1071 Specified key was too long; max key length is 767 bytes (localhost:/tmp/quibble-mysql-5u5evtc_/socket)
```