show formatted duty expression

The UK Trade tariff currently displays the measurement units "as is" from the TARIC database, some of which are quite old or long. There is a document published by the EU on how these units can be reimported in an abbreviated (and more common form). This PR implements the unit abbreviations and will change how the measures are displayed.

This PR compliments alphagov/trade-tariff-backend#189

[pivotal story](https://www.pivotaltracker.com/story/show/75057154)