Introduce common JavaScript file (#2981)

* Create common chunk rather than vendor chunk

vendor chunk is a set of modules provided by external vendors, but now we
can have a chunk as a set of modules shared by multiple entry points,
which could be more efficent than having vendor chunk.

* Start rails-ujs in common.js

This is used by /settings/two_factor_authentication.

app/javascript/packs/common.js

@@ -0,0 +1,2 @@
+import { start } from 'rails-ujs';
+start();