Add Spree::Config[:max_quantity]

This limits the amount of any one item that can be purchased in Spree.

Related to #2695

Fixes #2826
Making newly entered payments invalidate previously entered payments so the most recent payment is used by the gateway.

Fixes #2828
Use taxon name on taxons/show rather than its accurate_title

Fixes #2814
Factor product scopes to allow simple scopes to be added from an extension

Fixes #2608
Fixes styling issue where email is pulled left out of the info area.

Fixes #2850