Add commodity synonym management API endpoint

This change is for https://www.pivotaltracker.com/story/show/58810758

Adds synonyms on commodities. Been waiting to raise this PR until https://github.com/alphagov/trade-tariff-backend/pull/128 was merged in.

This depends on:

* https://github.com/alphagov/trade-tariff-admin/pull/17
* https://github.com/alphagov/trade-tariff-frontend/pull/111