Add an address multi-field partial

[pascallaliberte]

Create an address field that:

• includes all fields for an address: country, postal code, address lines 1 and 2, city, region (state, province, prefecture, etc)
• has the region and postal code field update themselves depending on the country selected
• is stored in a separate table, using has_one :billing_address, as: :addressable or something similar allowing for multiple such polymorphic fields per model