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/duration/duration.js

@@ -0,0 +1,16 @@
+// Side effect imports
+import './prototype';
+
+import { createDuration } from './create';
+import { isDuration } from './constructor';
+import {
+    getSetRelativeTimeRounding,
+    getSetRelativeTimeThreshold,
+} from './humanize';
+
+export {
+    createDuration,
+    isDuration,
+    getSetRelativeTimeRounding,
+    getSetRelativeTimeThreshold,
+};