style: enable shadows

I find it less confusing when using the suggest dropdown on the search page: the dropdown border doesn't "fuse" with the cards border that are located below.

frontend/src/styles.scss

 $fa-font-path: '../node_modules/font-awesome/fonts';
+$enable-shadows: true;
 
 @import '../node_modules/font-awesome/scss/font-awesome';
 @import 'custom-bootstrap';