Refactor ModelAsController to be generic

[GuySartorelli]

ModelAsController is currently hardcoded to work with SiteTree. It's not uncommon to want to route to a DataObject that isn't managed in the site tree, and therefore isn't a SiteTree object - but for the frontend to treat it the same way it treats SiteTree objects.

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

Acceptance Criteria

• Make this controller work with other DataObject subclasses
• Add a config property for the default model class - or move that logic into a different class
  • This is needed for logic in RootURLController
• If needed, remove assumptions that records will have the Versioned extension, or throw an exception if they don't.
• If necessary, define an interface or add explicitly hasMethod()/hasField() checks, and throw exceptions if needed fields/methods aren't available
• Either also make OldPageRedirector generic, or else only use it when SiteTree is the model being used.

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 ContentController and RootURLController)?
• Do we want to keep these here, and move SiteTree and CMSMain to a new silverstripe/pages module?