[WIP][FEATURE] added activities to account and contacts

__Tasks:__

- [x] datafixtures and entities for activities (status, type, priority)
- [x] controller / rest api
- [x] integration in UI (contact, account)
- [x] test coverage
- [ ] gather feedback for my changes

__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | yes
| Fixed tickets | none
| Doc           | none

__Requires:__
- newest husky version
- https://github.com/sulu-cmf/SuluContactBundle/tree/feature/rest-api-refactoring
- load new fixtures

__Issue:__
- https://github.com/sulu-cmf/sulu/issues/141
would it make sense to use constants for return codes?