Align on Out of Stock button logic

[dan-menard]

The logic we use for identifying when to replace a button with an "out of stock" notice varies a little across Dawn. Specifically, the way we do this on Quick Order List and Quick Add doesn't match how we do it for buy button.

We should figure out which of these is correct and update the other, or whether they actually need to be different to satisfy different use cases. If we don't do this, we might have inconsistent out of stock handling in different parts of the theme.