Prevent access to non declarable commodities.

This does not have a story, but has a ZenDesk ticket #63221. https://www.gov.uk/trade-tariff/commodities/3903909000 Should not be accessable as 3903909000 is an intermediate line, not a declarable commodity see https://www.gov.uk/trade-tariff/headings/3903. It's either that a search engine indexed these, or someone tried to enter code manually.

Children/ancestor code is pending refactoring, but I'm not sure how to do that better at this time, these codes do not follow any logical sequence.