Consider replacing Font Awesome by package dependency

[bittner]

As mentioned by @wherget in #39, Font Awesome offers fontawesomefree on PyPI, an official Python package for Django (see the official docs). Hence, it might be worth:

1. removing Font Awesome from this package and
2. directing our users to the official FA integration for Django.

To keep things convenient, we could add FA as an optional dependency (aka extras), so that it could be installed along with django-bootstrap-static as a single dependency. Installing would then look like this:

pip install 'django-bootstrap-static[fontawesome]'

Things to do

• [ ] Remove fontawesome module (aka folder) from Python package.
• [ ] Remove FA license file from project root.
• [ ] Add [project.optional-dependencies] section to pyproject.toml.
• [ ] Adjust usage instruction in project README, also pointing to FA's installation and usage instructions.