Shared money input should respond to change in currency (#654)

* Add step method to currency

* Change amount placeholder and step, when currency select change

* Lint

* Add test with auth

* Extract request to specific service

lib/money/currency.rb

@@ -58,6 +58,10 @@ class Money::Currency
         @default_precision = currency_data["default_precision"]
     end
 
+    def step
+      (1.0/10**default_precision)
+    end
+
     def <=>(other)
         return nil unless other.is_a?(Money::Currency)
         @iso_code <=> other.iso_code