Added database schema creation and deletion

This functionality is provided via the following routes:

  CREATE /createschema
  DROP   /dropschema

Notes:
  1. The tables of the AppointmentProposalDataAccessService are not created at the moment. Support will be added as soon as the component's implementation is finished.
  2. There seems to be a bug in slick regarding compound primary keys. Therefore the compound keys are commented out in CalendarDataAccessService.