[Example] Add custom icon font to the sulu admin build

[Prokyonn]

What's in this PR?

This PR demonstrates how you can add your custom icon-font to the sulu admin.

The example implements a custom icon font generated with the IcoMoon App. The custom font adds a Rocket icon. For the demonstration the new icon is used in the navigation for the Music item.

Steps

1. create your custom icon-font with the IcoMoon App
2. custom icons must be prefixed with su-, in the example the su-app- prefix is used

Settings

4. add the generated font to the project and add the css file of the font in your app.js
5. update the administration interface frontend application
6. use the new icon