Feature/8323 gradle migration 2

Completes support for Gradle builds - some functionality was merged in before the switch over to GitHub, so no PR was issued for that work.

What's here is is basically a 1:1 mapping of the Maven build into Gradle, and it will be modified moving forward to support a more tailored build process, and to make better use of Gradle idioms.  The Maven POMs have all been left in place, and the build is fully functional in that context.  There are a few places where we've had to create Gradle-friendly versions of resource files to allow for the Maven build to be supported in parallel, but these will be cleaned up once the Gradle build is deemed stable enough.

The `build.gradle`, `settings.gradle` and `gradle.properties` files are in the top-level `aperture-tiles` project and should act as the starting point for review.