Also modifies the `databases` example so that it makes use of the new
ability to run migrations in diesel-async v0.5. To accomplish this,
`diesel_async::async_connection_wrapper::AsyncConnectionWrapper` is
exported from `rocket_db_pools::diesel` and used in the `diesel_mysql`
portion of the `databases` example. The URL for the MySQL version of the
database example is now `/mysql` instead of `/diesel-async`.