Do not use "createdAt" to sort events by date

Currently, the "createdAt" property of an event is used to sort events. This has the advantage to make a fast search because there is no need to iterate inside the event versions. But this is a temporary fix because "createdAt" may not represents the moment the user created the event. See more in #66 (closed).

The idea is to have a field directly accessible in the last version of the event which gives the creation date of the original event.