Travel advice alert status should be enum or removed

[fofr]

• The alert status feature of travel advice isn't actively used, it could be removed from the publisher, schema and frontend
• Alternatively, the alert status is set to one of a defined set of values, it should be an enum locked down to just those values.

https://github.com/alphagov/travel-advice-publisher/blob/5650462b5d11b7cc86d95c2f4e4470c8eaf71c9f/app/models/travel_advice_edition.rb#L30-L35

https://github.com/alphagov/govuk-content-schemas/blob/master/formats/travel_advice/publisher/details.json#L33-L37