[enhancement] Fix bootstrap import in the theme

[zetxek]

Describe the bug

Currently the bootstrap files are modified by hand to integrate into the theme, by renaming the "vendor" folder:

adritian-free-hugo-theme/assets/scss/bootstrap/_mixins.scss

Line 6 in 64928b6

@import "bs-vendor/rfs";

This is wrong- it was just the "quick and dirty" fix for the encountered issue where "vendor" is a reserved folder name: #166 (comment)

As suggested by @mnordhaus in #165 (comment) we should look into a better way to integrate the dependency, following bootstrap advice.

The idea would be to use package.json for the vendoring.
A requirement is that the theme should still be usable "batteries included" as a git submodule (https://github.com/zetxek/adritian-git-submodule-demo can be used as a demo/test environment).