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

__Tasks:__

- [ ] datafixtures and entities for activities (status, type, priority)
- [ ] controller / rest api
- [ ] integration in UI (contact, account)
- [ ] test coverage
- [ ] gather feedback for my changes

__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | yes
| Fixed tickets | none
| Doc           | none