JENKINS-16361 Grab support

There's a few build changes and moving to jsp jelly changes in here too, all required to support @Grab. But the real magic is the Mask-Classes and PluginFirstClassLoader in the job-dsl-plugin/build.gradle. Thanks to @daspilker for the unit tests and doing the initial research. I had to test with a live instance to find some errors, hence we don't have tests for everything, but that's ok as far as I'm concerned.