Check for duplicate short codes

[vnugent]

We should enforce "airport-code" shortCode uniqueness within a country.

For example, there should only 1 CRACKS in the US.

Implementation ideas:

• [ ] add a new country code field to metadata. See addCountry()
• [ ] create a compound index with metadata.shortCode + metadata.countryCode

[CocoisBuggy]

I can work on this, but I have an architectural query:

Do shortcodes need to be globally unique, or do they need to be unique per root area \ parent area.

I can enforce the database rule easily enough, but would like to be sure I know how this system is meant to work 🤗