Find the available current or future bet events.

BetEventRepository is extended with the findAllAvailableEvents function
to fetch the available current or future bet events.
RestController's getBetEventsAction the findAll method is replaced with
the above repository function.
EntityBase modified to not show the created and updated properties for
the serialisation.
Bet fixtures created.

Bet event, bet slip, slip type, taken bet event fixtures are created.
Abstract data fixture class is created in the core bundle.