Added resource locator strategy to portal configuration

The portal configuration includes now a field for the resource locator strategy, which can be `tree` or `short`

- [x] test coverage
- [x] finish the code
- [ ] gather feedback for my changes


| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | yes
| Fixed tickets | non
| Doc           | none