Fix fa-content function and $fa-var variables

Fix to `_functions.scss` is trivial

As per `_variables.scss`, a “find and replace in selection by regexp”
functionality may be used.

1. Select lines containing variables
2. Find `: \\(.+);` and replace with `: "$1";`

Note: regexp replacement syntax may change according to the editor.

Our goal is to turn `$fa-var-0: \30;` into `$fa-var-0: "30";`

app/javascript/stylesheets/_functions.scss

@@ -3,7 +3,7 @@
 
 // fa-content: convenience function used to set content property
 @function fa-content($fa-var) {
-  @return unquote("\"#{ $fa-var }\"");
+  @return unquote("\"") + unquote(str-insert($fa-var, "\\", 1)) + unquote("\"");
 }
 
 // fa-divide: Originally obtained from the Bootstrap https://github.com/twbs/bootstrap