Upgrade to Play 2.3 [finishes #72619526]

* Remove requireNativePath from Build.scala
* Pass JS engine arg to Play from Runner
* Use Node over Trireme if possible
* Remove root-level r.js (Node) package.
* Remove startHelper. We haven't used it in ages.
* Remove less-compiling stuff. Play 2.3 obviates it.
* Remove unused ClearTestDatabase.scala
* Remove SimpleDist.scala
* Fix race in ExampleDocumentSetSpec