Support for Django-Sass/Compiling Bootstrap 4 from Sass

[TheRadialActive]

For now we're using the static .css-files of bootstrap 4. We should compile bootstrap CSS from the Sass/SCSS files. Then we can add new color schemes easily (I would like to see the 'tango icons' color scheme ;) ).

Django-Sass can help us with that: https://pypi.org/project/django-sass/