[FEATURE] added functionallity to convert accounts

added configuration

__Tasks:__
- [x] integration in ui
- [x] api adjustments
- [x] test coverage
- [ ] gather feedback for my changes

__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | yes
| Fixed tickets | https://github.com/massiveart/POOL-ALPIN/issues/9
| Doc           | none

__Dependencies:__
- https://github.com/sulu-cmf/SuluAdminBundle/pull/119
- https://github.com/sulu-cmf/sulu-standard/pull/84
@wachterjohannes fixed in next commit