Generate DatabaseCleaner setup in new apps

[timriley]

Cleaning the database between tests is essential functionality, and I don't think users should have to think about it.

We need a setup that:

• Works with regular tests (transaction) as well as capybara-with-JS tests (truncation)
• Works with multiple databases across slices

I think we should just generate the appropriate config into spec/support/database_cleaner.rb for the app database, and include some commented examples of how to make it work for additional databases.

Later (post-2.2) we can look at ways at codifying certain aspects of iterating over/interacting with the app's set of databases into hanami-db, and then find ways for that to be used to configure DatabaseCleaner.