Collapse imports in code examples in Javadoc and manual

[ldaley]

It would be nice to not show the imports by default in the code samples. We should replace this with some kind of toggle thing that expands / collapses the imports.

Would need to add this to the Javascript we already pull in to do the syntax highlighting.

[ldaley]

Work has started on this: https://github.com/zedar/ratpack-manual-toggle-imports

Thanks to @zedar.

[zedar]

#526
Pull request contains suppor for hiding/showing import statements in code samples in manual. It does not contain functionality for Javadoc API samples. Javadocs have different DOM structure. Will add missing functionality in the next pull request.

[zedar]

#527
Merged changed to existing manual css and js files.