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

app/javascript/services/currencies_service.js

@@ -0,0 +1,5 @@
+export class CurrenciesService {
+  get(id) {
+    return fetch(`/currencies/${id}.json`).then((response) => response.json());
+  }
+}