Warn when ordering excessive quantities?

[julian-git]

We should think about warning people when they order unusually high quantities.
This could mean (a) more than 5 if the order_unit_measure is "kg" or (b) exceed the average consumption today by 200% if that is non-zero, among other things;
also, we'd have to think about when this is going to fire:

• a listener in each quantity box? (expensive DOM)
• before sending the query to the database, ie in a validation step. This involves another query if we use the average consumption; otherwise, for method (a), an entry in config.php for each unit_measure. so maybe tha't the best way to go...