Add currency to account form to support multiple currencies (#481)

Co-authored-by: Sriram Krishnan <sriram@seafoodsouq.com>
Co-authored-by: Zach Gollwitzer <zach.gollwitzer@gmail.com>

app/models/exchange_rate.rb

@@ -1,5 +1,7 @@
 class ExchangeRate < ApplicationRecord
   def self.convert(from, to, amount)
+    return amount unless EXCHANGE_RATE_ENABLED
+
     rate = ExchangeRate.find_by(base_currency: from, converted_currency: to)
     amount * rate.rate
   end