Package assests on repository instead of using CDN

At the moment, we plan to use CDNs for things like bootstrap and fontawesome.

In merge request #1, we started discussing the idea of bundle this kink of resources during the build. This would give us more control over our delivery of assets.

As a starting point, we could follow a tutorial like:

- [How to Use ES6 and Webpack With Jekyll](https://michaelmovsesov.com/articles/jekyll-es6-workflow)

Original thread [here](https://github.com/cesium/HeartBits2020/pull/1#discussion_r432993128).