extend new employee query

Extend the new employee query, so that rooms are listed that currently may be occupied, but become available by the time the new employee starts.

Therefore you need to add a new query parameter (analog to :rolle or :sektion), e.g. :start

When the occupied space for each room is calculated, you have to consider the contract duration for each employee and only count them in in case the contract ends after the start of the new employee. Shouldn't be that hard to include into the query.