Transformed site to Bootstrap 4

See #11

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

_assets/css/_bootstrap-custom.scss

@@ -8,6 +8,10 @@
 @import "bootstrap/functions";
 @import "bootstrap/variables";
 @import "bootstrap/mixins";
+
+// overwrite variables
+$body-bg: #fdfdfd;
+
 @import "bootstrap/root";
 @import "bootstrap/print";
 @import "bootstrap/reboot";