Add font-awesome-sass gem

Now can use all icons from https://fontawesome.com.

app/assets/stylesheets/application.scss

@@ -1,6 +1,8 @@
 @import "social-share-button";
 @import "foundation_and_overrides";
 @import "fonts";
+@import "font-awesome-sprockets";
+@import "font-awesome";
 @import "icons";
 @import "mixins";
 @import "admin";