[WIP][FEATURE] integration of documents

__Tasks:__

- [x] extend database and orm
- [x] extend controller
- [ ] implement tests
- [x] integrate in UI (contacts)
- [x] integrate in UI (accounts)
- [ ] test coverage
- [ ] gather feedback for my changes

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | none
| BC Breaks     | none
| Doc           | none

__Requires:__
- https://github.com/sulu-cmf/SuluMediaBundle/pull/55
- https://github.com/sulu-cmf/SuluMediaBundle/pull/57
- https://github.com/sulu-cmf/SuluAdminBundle/pull/146
@michaelzangerle perhaps slug has different meanings! use mediaId and contactId (accountId)