Make available quantities display more configurable

The "Only X left" display on products needs to be more configurable (Product > Settings > Only X left Threshold).

Instead of a single global setting we'd like to see the setting on each product. Some items are high volume products and some are low volume, some items we might want to show available quantities but not others, so the current one-size-fits-all approach does not fit well.

We'd also like the ability to enable this by customer class. We might not want to display inventory quantities publicly, but would like to show them to a private, pre-approved customer class, like our wholesale customers for example.