Refactor Lagom build description

[ignasi35]

(This is to discuss improvements and link to small PRs)

The build for Lagom itself (build.sbt and projects/*) is quite convoluted code and some basic maintenance can sometimes become a tedious or error-prone task. (see #1493 and #1494 edit and #1497 for example).

It'd be good to have a smaller, cleaner build.sbt by applying some ideas from other builds this size such as Akka and Play:

1. In Akka, there are custom sbt plugins extending upstream equivalent and overriding the default for Akka's needs.
2. In Play, there is a collection of factory methods specialised on each type of Project the Play build needs.

[erip]

This will be nice for when we enable the SBT 1.x scripted plugin, because it requires non-scripted projects to disable the plugin, which is most of them in the case of lagom.

[ignasi35]

related: #1474