refactor user manager

Discussion: https://github.com/sulu-cmf/SuluContentBundle/pull/17#discussion_r8694209
Resulted in this PR

- [ ] test coverage
- [ ] finish the code
- [ ] gather feedback for my changes
- [ ] submit changes to the documentation


| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | yes
| Fixed tickets | none
| Doc           | none