add `bg-body` class

Created by: skeets23

text-body exists, but bg-body is missing from Bootstrap. It's commonly needed... for example, with absolute / fixed position elements that overlap other elements, or just when you have one element with bg-success or something and you need a "plain" background element inside it.

It's super easy to add, but I've had to add it to every project I've ever used bootstrap in. It just seems like it would make sense to be there to begin with.