Pass accessory's currency if set by location

[snipe]

This sets the override for an edited accessory if the accessory has a location and that location has a currency.

[what-the-diff]

PR Summary

• Enhanced purchase_cost Form
  The form on the edit page (where you can change information about items) has been improved to include an additional piece of information, the currency of the item cost, according to its specific location. You can now see item prices in their original currency.