boolean to indicate sales tax exemption support for offers

[fthobe]

This is a first step to incorporate taxes into offers laying the ground work to define acceptance of sales tax reductions.

In many jurisdictions there are several reasons to be exempted from sales tax. Below some examples:

• in many countries people with disability receive sales tax / vat discount (eg Italy blind people 4% instead of 22%);
• the entire European union supports VAT exemption for businesses dealing directly (eg one company selling a service to another);
• in the US sales tax can be exempted in many cases between businesses.

This is achieved through two booleans on offers:

1. acceptsTaxExemptionsDisability allows businesses to markup disability sales tax reductions;
2. acceptsTaxExemptionsBusiness allows businesses to markup B2B sales tax reductions;

This PR introduces the ability to markup sales tax exemption support for merchants.

Concerns (but does not solve): #380

[fthobe]

@alex-jansen this is a proposal to add some groundwork for VAT (the ability of the merchant to process orders without VAT / Sales tax).

Examples would be:

• Amazon Sales Tax Exemption Program

• Candle Science

• EU has a compulsory program

It would be for me a sensible first step, I have some questions though:

1. locally sales taxes have different names (VAT in Europe, GST in India and many Asian countries, Sales Tax in the US), how could we account for the differences in naming this?
2. What would be a good comment in your opinion?

It's a very atomic change, should it never the less go into pending first?

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