Let Jekyll transpile Sass

Jekyll automatically converts Sass files in place if they contain front
matter. Moved `main.scss` and `noscript.scss` to `/assets/css` so that
the output can be reached via `/assets/css/main.css` and
`/assets/css/noscript.css`, respectively. This enables editing of
stylesheets, as well as the use of Liquid tags in aforementioned files.
The old css files are not needed anymore.

Jekyll tells Sass to look in `_sass` (configured by
`site.sass.sass_dir`) when it reaches an import. The remainder of
`/assets/sass` was moved there. According the docs, Liquid tags don't
work in this directory.

Exclude `.sass-cache` form the repo.

https://jekyllrb.com/docs/assets/

.gitignore

@@ -1,3 +1,4 @@
 _site
 _site/
 .DS_Store
+.sass-cache