[BUGFIX] Grouped Assets with Themes

Grouped all the assets (javascripts, fonts, images) with the themes.

__Tasks:__

- [ ] gather feedback for my changes
- [ ] submit changes to the documentation

__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| Tests pass?   | no
| Fixed tickets | none
| Doc           | https://github.com/sulu-cmf/docs/commit/18d22d0682d12d66f3b262950b3b43e99d6065ba