Add TradeTariffBackend::NumberFormatter

Helps format duty expression amounts. Cases:

* If we have 0.3435 we want this exact number => 0.3435
* If we have 0.34528 we want it rounded to 4 precision points => 0.3453
* If we have 0.23 we want to leave it as it is => 0.23
* If we have 0.2 we want add single leading zero => 0.20
* If we have 1 we want to add two leading zeros => 1.00
Adjust national measure creation

For Excise measure with tty codes 570 and 551 we should create a measure
with rate of 0, measurement unit Litres and monetary unit GBP.
Add data migration to fix national measures

..With tty codes 570 and 551
Merge pull request #124 from alphagov/59551288-chief-excise-measure-type-fix

CHIEF Excise measure type fix