Fix MariaDB data removal

Finally working to resolve #853008 by adding checks for MariaDB on install and removal.

When installing mysql-server, the preinst matches mariadb-server's behavior by checking for a debian-*.flag file to check if MariaDB exists. If so, it moves the data to a new location in the same way, and shows the info in the attached image.

When removing mysql-server-8.0, the postrm checks again for MariaDB in the same way. If it is detected it notifies the user. On a purge the files are not removed when MariaDB is detected.