jQuery and Bootstrap: allow to host locally with env options

Introduce the following environment variables:

• JQUERY_URL_JS
• BOOTSTRAP_URL_JS
• BOOTSTRAP_URL_CSS

This change keeps backward compatibility.

For example, to work locally, you can now just define these in your .env file:

JQUERY_URL_JS=/js/jquery-3.3.1.min.js
BOOTSTRAP_URL_JS=/js/bootstrap.min.js
BOOTSTRAP_URL_CSS=/css/bootstrap.min.css

Closes #97