Add lagomLogback dependency and a logback.xml sample

[Ignasi Marimon-Clos]

It'd be good to have logback enabled and configured by default on the seeds.

[Tim Marcus Moore]

lagomLogback is added automatically by an sbt auto-plugin:

https://github.com/lagom/lagom/blob/ea8b417e9b520a6f66370be87dd3cf9d17a4c6a0/dev/sbt-plugin/src/main/scala/com/lightbend/lagom/sbt/LagomPlugin.scala#L690-L699

Lagom provides a default logback.xml configuration inside the framework JARs, so it isn't strictly necessary to have one inside the project..

That said, I do think it's worth including logback.xml explicitly in the templates, because right now it's totally invisible. The problem is: Lagom's default behavior uses slightly different configurations in development and production modes, but this isn't easy to replicate with an explicit config file in the project.