Added VAT handling through services & persistence

Introduces a `VatService`, that uses a `VatHandler`, with a legacy implementation.

Adds an `ezprice_value` twig function that transforms a `Price\Value` to a `PriceWithVatData`, that aggregates the original price data with `priceIncludingVat` and `priceExcludingVat` properties.

### TODO
- [ ] Change the `VatService` so that `loadVatRate()` doesn't take a field id and a version number. It should take a vat ID.
- [ ] Remove the logger mock from `PriceExtensionTest` (unless we actually add expectations to the call so that it asserts that data is actually logged when expected