Use sbt-header to add apache-2.0 license notice to source files

[adriaanm]

Prior work in akka-land: akka/akka-http#1738 (and others in akka/akka).
Prior work in scala-land: scala/scala#5145

[jvican]

This is how we do it in sbt/zinc: https://github.com/scalacenter/zinc/blob/1.x-bloop/project/Header.scala The license message can be tweaked.

[dwijnand]

Fixed by scala/scala#7306