Multi-currency support (#425)

* Initial foundational pass at multi-currency

* Default format currency

* More work on currency and exchanging

* Re-build currencies on change

* Currency import/setup

* Background job overhaul + cheaper OXR plan support

* Lint fixes

* Test fixes

* Multi-currency setup instructions

* Allow decimals in the balance field

* Spacing fix for form

---------

Signed-off-by: Josh Pigford <josh@joshpigford.com>

app/models/account.rb

@@ -5,5 +5,15 @@ class Account < ApplicationRecord
 
   delegate :type_name, to: :accountable
 
-  monetize :balance_cents
+  before_create :check_currency
+
+  def check_currency
+    if self.original_currency == self.family.currency
+      self.converted_balance = self.original_balance
+      self.converted_currency = self.original_currency
+    else
+      self.converted_balance = ExchangeRate.convert(self.original_currency, self.family.currency, self.original_balance)
+      self.converted_currency = self.family.currency
+    end
+  end
 end

app/models/currency.rb

@@ -0,0 +1,2 @@
+class Currency < ApplicationRecord
+end

app/models/exchange_rate.rb

@@ -0,0 +1,6 @@
+class ExchangeRate < ApplicationRecord
+  def self.convert(from, to, amount)
+    rate = ExchangeRate.find_by(base_currency: from, converted_currency: to)
+    amount * rate.rate
+  end
+end