Use gross amount in return items

[mamhoff]

I believe the notion of pre_tax_amount is only present in the return items because
someone from a sales tax country wanted to use something excluding sales (additional) taxes.

This leads to unfortunate behaviour in VAT stores: While every other price or amount
is entered including VAT, for returns a store admin has to enter the return amount
excluding VAT. For most normal people, that's a complex, error-prone op, and one we
shouldn't make them do.

This PR replaces the pre_tax_amount with one called amount that behaves like the amount on line items: including included taxes, but excluding sales taxes.

Unfortunately, this entails a lot of naming changes.

This ports spree/spree#6852 to Solidus.

It's one large commit because all the naming changes affects everything :(

[jhawthorn]

I believe the notion of pre_tax_amount is only present in the return items because
someone from a sales tax country wanted to use something excluding sales (additional) taxes.

I believe the opposite to be true, pre_tax_amount was introduced purely for VAT countries (whether it's appropriate or helpful there is another discussion). When applying taxes without included rates pre_tax_amount == amount.

I'm 👍 and this looks good, but we'll need to be very clear in our release notes about what the behaviour change is as it will affect admin end-users.

[mamhoff]

The reason I believe that pre_tax_amount was introduced from a sales tax country perspective is that the form field for entering the data was labeled pre_tax_amount, indicating that the admin user should enter the reimbursement excluding sales tax, as sales tax would be added later. In a VAT country, I can not imagine the admin user actually wanting to do essentially this calculation in her head:

I want to reimburse 10 Euros. 
My local VAT rate is 19%. 
10 / 1.19 -->  8.40 
OK, I enter 8.40 in the form field. 

What about this as a release notes approach:

When creating a refund for an order including VAT, you now have to enter the amount you want to refund including VAT. 

The form field and the permitted attribute for the return item amount changes from `:pre_tax_amount` to :amount`. 

[cbrunsdon]

do we need to be worried about included_tax_total ever being nil? I'd had to nullify pre_tax_amount from included_tax_total being nil. Should we switch to WHERE pre_tax_amount is null to be safe?

[mamhoff]

I added a safeguard clause that sets included_tax_total to 0 if NULL. The pre_tax_amount/ amount column has a NOT NULL db constraint set, so no worries on that side of the plus.

[cbrunsdon]

I'm 👍 on this, sorry, missed the feedback to my migration question. Hope the rebase isn't too brutal... cough