Added a new Vat Finder Service and refactored the Vat Service to work wi...

...th the vat identifier instead

**WIP** 

* Added the new VatFinderService. It has a method to get the vat id from the field 
* The VatService one is refactored and the load logic only needs the vat identifier to get the data
* Modified twig template accordinglly

Test are failing, fixing them now.
Could we do this earlier, and never send -1 to `getVatRateData` ? -1 really has a particular meaning, doesn't it ?