Option to include tax in item prices

[derBretti]

In many places with a unified VAT, it is common to display item prices with taxes included if the shop is mainly intended for private customers. I see a few reasonable ways to implement this:

1. Let the customers choose how they wish prices to be displayed (might require quite a few changes)
2. Add an option to the shop settings to choose how prices should be displayed (tax calculation for items in an order and calculation of the tax summary need to change)

For a discussion of implementing the option per tax rate, see https://github.com/reactioncommerce/reaction/issues/3342

[derBretti]

For those interested in the feature: I implemented it along with a few other feature related to the treatment of VAT, see: https://github.com/derBretti/reaction/tree/feat-derbretti-VAT-features I am happy for comments and PRs