Add support for custom classes for mutli-field panels + optionally make them collapsible

I've removed a lot of trailing whitespace, see last commit for the relevant changes.

This allows you to specify classnames for MultiFieldPanels as well:

MultiFieldPanel(COMMON_PANELS, "Common page configuration", classname="collapsible collapsed")

The collapsing behaviour is triggered by these classnames and works with MultiFieldPanels only.