Add db rollback command

[timriley]

$ bundle exec hanami db rollback # Rolls back app database
$ bundle exec hanami db rollback 3 # Rolls back app database
$ bundle exec hanami db rollback --app # Rolls back app database
$ bundle exec hanami db rollback 3 --app # Rolls back app database
$ bundle exec hanami db rollback --slice=billing # Rolls back "billing" slice database
$ bundle exec hanami db rollback 2 --slice=billing # Rolls back "billing" slice database

Note: this already exists as a command, but it has been disabled for a while. It just needs to be reactivated, and updated for the per-slice behaviour.