Revert "Use jekyll-assets"

This reverts commit 9062937951.

css/main.scss

@@ -0,0 +1,14 @@
+@charset "utf-8";
+
+@import "partials/defaults";
+@import "partials/colors";
+@import "partials/helpers";
+
+@import "modules/base";
+@import "modules/layout";
+@import "modules/buttons";
+@import "modules/home-banner";
+@import "modules/home-users-grid";
+@import "modules/content-area";
+
+@import "vendors/syntax-highlighting";