Get off off rails-assets

Just moving the things we currently use (and should drop in the ASAP) to vendor/assets

vendor/assets/javascripts/momentjs/src/lib/utils/is-number.js

@@ -0,0 +1,6 @@
+export default function isNumber(input) {
+    return (
+        typeof input === 'number' ||
+        Object.prototype.toString.call(input) === '[object Number]'
+    );
+}