DRY up address attributes and validation, correct some field length validation according to Jadu spec
Minor adjustments to AddressAttributes

 - Use ActiveSupport::Concern API for DRYness
 - Rename TOWN_COUNTY_LENGTH -> LOCALITY_LENGTH to match our schema
 - Alphabetically sort constants in Form
Merge pull request #64 from ministryofjustice/address_refactor

DRY up address attributes and validation, correct some field length validation