Extract duty expression and description formatting

This change is for https://www.pivotaltracker.com/story/show/36718669 and partly covers https://www.pivotaltracker.com/story/show/58242830

Should be merged with pull request on backend app: https://github.com/alphagov/trade-tariff-backend/pull/122

**N.B.**, this is not an epic pull request, VCR mock response updates are to blame for extreme line count in the diff. I mostly removed code from this app.

Formatting was moved to backend app, so it can serve formatted data via API so that other clients wouldn't have to know TARIC rules to display the data.

Updated `therubyracer` to avoid compilation errors