Simplify how scripts get icon slugs (#5066)

* Accept icon rather than title in `iconToSlug`

This removes the need in (most of the) calls to this function to
seperately check if there is a custom slug already defined for the icon.

* Rename `titleToSlug` to `getIconSlug`

* Revert unnecessary style change

* Update function documentation

* Keep `titleToSlug` and add `getIconSlug` helper

Unfortunately `this` is not available because we're using arrow syntax
for these functions, but refering to `titleToSlug` through
`module.exports` works fine as well.

* Fix parameter name in `getIconSlug` docs

scripts/utils.js

@@ -4,6 +4,12 @@
  */
 
 module.exports = {
+  /**
+   * Get the slug/filename for an icon.
+   * @param {Object} icon The icon data as it appears in _data/simple-icons.json
+   */
+  getIconSlug: icon => icon.slug || module.exports.titleToSlug(icon.title),
+
   /**
    * Converts a brand title into a slug/filename.
    * @param {String} title The title to convert