Added country_supplies join table and associates

A link was created between countries and supplies via country_supplies.
Added migration to populate country_supplies for all items/countries
Added self.choices_for to Supply
Updated view to use Supply.choices_for
Updated choices_for to return all if arg is nil
fixed failed specs due to lack of country/supply relationship
Incoming shortcodes now must be country specific.

Incoming shortcodes must now be valid for the specific country
of the requester, else they are invalid and raise an exception.