[FEATURE] PortalLoader for Routes

Added a route loader for creating routes for portals

__Tasks:__

- [x] test coverage
- [x] gather feedback for my changes
- [x] submit changes to the documentation


__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | no, because of missing merge https://github.com/sulu-cmf/sulu/pull/164
| Fixed tickets | none
| BC Breaks     | May override already existing routes
| Doc           | https://github.com/sulu-cmf/docs/blob/master/developer-documentation/300-webspaces/portal-route-loader