Don't want to call it `Files` because there's already a really useful class called `Files` in guava.

Have added `ResourceUtils.mergeFilePaths(String...)` and updated a couple of places to use it.

However, places a couple of remaining places where it was just appending two things the code looked harder to understand using this method versus a simple append. It's worth it in places where we think the first part might or might end in "/" and we don't want a double slash etc.