Role creation command

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes
- [x] submit changes to the documentation
- [x] added changelog line
- [x] make name of role unique
- [x] show error label in form
- [x] implement role creation comannd
- [x] add question for role in user creation command


__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | fixes #466 and fixes #656 
| BC Breaks     | role is now unique (added to changelog and upgrade)
| Doc           | none
Symfony always uses `\DateTime`. Same for the CMF + Doctrine.