OptaShift: availability roster should outbound availabilities (as there can only be one at a specific point in time)

Notice how:

• A infeasible plan with 2 shifts at the same time for an employee still needs to be visualized. This can happen, although users don't like it. The UI needs to be able to deal with it.
• It can never happen that there are 2 EmloyeeAvailability at the same time (or overlapping): that would mean the database is corruption. The REST service should prevent this from occurring, by throwing an exception if a new EmloyeeAvailability would overlap with an other one of the same employee. Therefore, the UI doesn't need to deal with it.

So we can do this:

• span the EmloyeeAvailability to the entire height of the row.
• nest the shifts into the EmloyeeAvailability.
  • Notice that a shift might be partially or completely outside an EmloyeeAvailability.