Rework environment identification in alert emails.

As seen in whitehall, it seems that environment name can be fetched from
FACTER_govuk_platform environment variable.
Include dates in tariff alert emails.

Also abstracts mail prefix formatting.
Add chief_comm table.
Add COMM mapping for TariffImporter.
COMM is a relevant record to import now.
Fix ordering on COMM strategy. Add origin column to chief_comm table.
Add NationalMeasurementUnit.
Add GoodsNomenclature to NationalMeasurementUnit association.
Present NationalMeasurementUnits with the declarable.
Add table, model and importer mapping for TBL9 table.
Fix datasets. And add unit of quantity associations.
Additional CHIEF fields and scopes.
Extract CHIEF initial loader to its own class.
Insert records into TBL9 table if they are of UNOQ type.
Fix chief_comm decimals.
Associate goods to national measurement units through CHIEF tables.
Remove schema changes as migrations were removed.
Expose NationalMeasurementUnits properly.
These are not used at the moment.
Limit is not a proper criteria for dataset.