[WiP] Code snippet generator

[Trello](https://trello.com/c/8pek8IgT/123-code-snippet-generator)

Still working on getting the JavaScript mechanics right (help from @tfennelly might be needed); would really like to define the new form in `CpsFlowDefinition/config.jelly` and have it be moved upon page load outside the regular `config` form, but do not have this working yet. The current version forces `Job/configure.jelly` to be overridden in `WorkflowJob`, which is not maintainable, and there is no hook in the original whereby a subclass could insert content after the form. Alternately could define a `PageDecorator/footer.jelly` which somehow detects that a `CpsFlowDefinition` is being configured. Or perhaps embed an `<iframe>`. Nesting `<form>`s is prohibited by the HTML spec and indeed seems to cause mayhem, at least in Chrome.

Or could perhaps just add the `f:dropdownList` to the main `config` form under a dummy `name`, build the content of the main form, and simply ignore other parts.

The next step is to send the JSON to some endpoint in `CpsFlowDefinition.DescriptorImpl` which would return some text that should be copied to the clipboard.

Then the fun part: converting form JSON to Groovy code.