Convert invoices into Yen

[thogg4]

We would like to convert the invoices show page dollar values into yen. Please add a Convert to Yen button (mockup​ below) that when clicked will convert the dollar amounts into Yen amounts.

Notes:

• There is a class in app/services/currency_converter.rb that you can use to do the conversion like a client for an external API would.
• Yen does not use decimals and the yen sign postfixes the amount (eg 1000 ¥)
• We would like to avoid any changes to the database