NEW Upload->replaceFile setting

This was required for a client who wants to manage files a bit more tightly, and replace existing files with the same name by default rather than appending numbers and creating a new file.

Aside: I've used _config/config.yml because I think one config file per class is getting ridiculous. Sure, they should have names so we can override specific config aspects, but we should also give devs a realistic chance of understanding the existing config values available. Since we have zero introspection there at the moment (http://open.silverstripe.org/ticket/7827), I think fewer files are better.