make upload folder configurable

[wernerkrauss]

Either per config setting and/or method with hook, so we could overwrite it per pagetype. Would be great if it was compatible with https://github.com/wernerkrauss/silverstripe-folderperpage, at least with a wrapper.

I suppose it should look if owner has a method "getUploadFolder(type)" which returns the value for images, files etc...

If not take the config value.