Use Drupal.url() where possible.

Problem/Motivation

In 🐛 Drupal CMS Needs review we identified that our manually-constructed URL's based on base path, etc. don't properly handle language switching. Though we repaired the issue over there, it makes more sense to lean on existing Drupal JavaScript API's where possible to ensure we're not reinventing the wheel and causing bugs that don't need to be there.

Proposed resolution

Replace front-end usage for BASE_URL constant with Drupal.url(), where possible. If not possible, include comments about why it could not be done.