Got the Create Another button basically working.

To make the code more tractable, split the ThingEditor file out from
EditorModule.  This is really nothing more than a code split -- they're
still effectively one object.

Exposed the getInstanceEditor entry point, which is roughly the guts of
_edit.

Note beginnings of messing with Scala.js.  Didn't get far, though: need
Play 2.2 to use it.