Replace core non-deductible-amount field with a custom field

Motivation: Apparently sometimes core will put the full amount of the contribution including eligible parts into the non_deductible_amount field. Currently cdntax uses that field to allow someone to manually indicate an additional ineligible portion. So it then thinks none of the contribution is tax-receiptable.

The idea is to hide the core field and replace it with a custom field, and then ignore the core field. While existing values will get copied over on upgrade in case people are depending on it, there may need to be some communication about using the new field going forward, and/or an upgrade check of some kind to see if people are using the old field in their hooks expecting it to still represent a manual override.

Related: #126