Use Typesafe Config for database configurations

- JdbcBackend gets a new factory method Database.forConfig for loading
  a configuration from application.conf or a custom config object. There
  is no built-in pooling support yet but the basic configuration keys
  are compatible with Play’s database configuration.

- We also use Typesafe Config for TestKit so we don’t have to “parse”
  a custom properties file and perform substitutions on it anymore. All
  defaults and substitutions are now in testkit-reference.conf.

- The main TestKit configuration is loaded from test-dbs/testkit.conf
  which is merged with the reference configuration so you don’t have to
  copy any default values into your own config file. This makes the
  configuration more resilient to changes in the defaults.