Add event to admin

## Concerns
- Event need be associated to organizations.
- Event need have tags.
- Admins need be able to approve events to be public.
- Admins need be able to cancel events.

## Todo
- [x] Create event model.
- [x] Add state machine. (pending, visible, canceled)
- [x] Create admin for events.
- [x] Create view for showing the event.
- [x] Add option to reject, cancel and approve events on admin.
- [x] Add association to organizations.
- [x] Add image uploader.