Enhancement: Use Doctrine migrations for database schema changes

This PR

* [x] adds `doctrine/orm-module`, `doctrine/migrations` and `symfony/console` as dependencies (with versions that don't break the console commands
* [x] enables `DoctrineModule` and `DoctrineORMModule`
* [x] adds default configuration to `conf/autoload/global.php` and placeholder configuration in `conf/autoload/database.local.php.dist`
* [x] splits the existing SQL dump into two migrations, the first one reflecting the state on the production server, the second one applying the necessary changes to fix #251
* [ ] updates `README.md`