[WIP] Portal Configuration

Should implement the loading of the portal configuration. The portal manager should be implemented as a service, which offers the data configured in the portal xml files.

- [ ] implement the entity classes completely
- [ ] finish portal manager (service)
- [ ] implement tests
- [ ] gather feedback for my changes
- [ ] submit changes to the documentation


| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | no
| Fixed tickets | no
| Doc           |