Added a clause to the add shift method that checks the availability and makes sure that the start is later than the end.

After a change in the database that made email addresses unique, caused a bug where new employees on the app to not be added after the first one.

Fixed a bug where isManager wasn't properly evaluated, and changed how positions in the employee are separated.

Added a claus that checks if a position is removed from all employees during the "removePosition" case.

Added comments, and added a method to receive the availability of a new employee. Also fixed a bug in the splitting of some data.