[FEATURE] PortalLoader for Routes

Added a route loader for creating routes for portals

__Tasks:__

- [ ] test coverage
- [ ] gather feedback for my changes
- [ ] submit changes to the documentation


__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| 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
We should write a phpunit extension for this (or use a base class, but that sucks)