Refactor RootURLController to be generic

[GuySartorelli]

RootURLController is currently hardcoded to work with SiteTree. It's not inconceivable that someone might want to route to a DataObject that isn't managed in the site tree as the "home" route.

Making this class more generic removes some friction from that use case.

Acceptance Criteria

• Make this controller work with other DataObject subclasses

TO DECIDE

Arguably this doesn't open much opportunity if we don't move some stuff around as well - but what should we move, and to where?

• Do we want to move this to silverstripe/framework?
• Do we want to move this to a new module (along with ModelAsController and ContentController)?
• Do we want to keep these here, and move SiteTree and CMSMain to a new silverstripe/pages module?