Also, add HTML for third dropdown list

Create parent `<div>` elements for both dropdown lists. In addition,
attach a barebones event listener to the new dropdown list

There seems to be conflict between the backend server (Node.js &
Express) and our front-end (browser-based) if we want to use module
imports and exports. It is not currently possible to use Node.js /
CommonJS modules. The current solution uses ES6 modules. May need to
look for a better approach.