Validate expense/reimbursement forms

Adds some general helpers and styles for minimalistic form validation.

app/utils/is-valid-amount.js

@@ -0,0 +1,8 @@
+export default function isValidAmount(inputAmount) {
+  const amount = parseFloat(inputAmount);
+  if (Number.isNaN(amount)) {
+    return false;
+  } else {
+    return amount > 0;
+  }
+}