Moved bootstrap to critical styles

Signed-off-by: Jim Martens <github@2martens.de>

_assets/css/critical.scss

@@ -1,2 +1,3 @@
 // contains critical css that needs to load immediately
+@import "bootstrap-custom";
 @import "fa-svg-with-js";

_assets/css/main.scss

@@ -1,7 +1,6 @@
 @charset "UTF-8";
 
 // Import partials from `sass_dir` (defaults to `_sass`)
-@import "bootstrap-custom";
 
 .site-header > .navbar {
   //border-top: 5px solid $dark;