Factor leave into calculations relative to contract hours

[LucaScorpion]

It's possible for an employee to have a couple of hours of maternity leave ("ouderschapsverlof"), which shouldn't be factored into the calculations. This is a fixed amount of time per week, which should - for the purposes of our calculations - be deducted from the contract hours. Currently, when using the "relative to contract hours" view, this is not done, so the billability percentage will seem lower than it actually is.

I'm not entirely sure what would be a good approach to factor this into the calculations, but it's something to think about.

[DannyvdSluijs]

We are now tracking parental leave (maternity leave is related to the 10-week period the mother gets after giving birth) as an activity called Ouderschapsverlof in Timechimp. This creates an opportunity to deal with the exception how these hours are handled compared to the other special leave.

As a side not in #15 we are adding Ouderschapsverlof to the LEAVE_TASKS to skip these hours in calculating the percentage relative to the amount of hours worked.

[LucaScorpion]

#15 is a partial fix, but the contract-hours-based view is still incorrect.

[LucaScorpion]

Just got another request for this feature. Currently it seems like the right approach would be to subtract the LEAVE_TASKS-hours from the contract hours for the billability calculation in that view. Will get on that when I have time!