This fixes the following deprecation warning
```
DEPRECATION WARNING: Extra .css in SCSS file is unnecessary. Rename /usr/src/app/app/assets/stylesheets/darkswarm/variables.css.scss to /usr/src/app/app/assets/stylesheets/darkswar
m/variables.scss. (called from _app_views_layouts_darkswarm_html_haml___3990351141569700945_46957256630860 at /usr/src/app/app/views/layouts/darkswarm.html.haml:18)
(...)
```
The route and controller were in the main app and the views in the engine, with this commit they stay all inside the engine
This is done to keep it simple and remove the unnecessary dependency between main app and engine
If we use this mechanism in the future for other things in the main app or other engines, we can find a way to extract/abstract this