Bill according to active reservations

This changes the billing to respect active reservations. 

It also changes the behavior of the resource usage page. This page used to just take a date range and calculate the prices for each flavor for this time range. Now it's splitting the time range into single days and does the calculation for each day and each flavor. This makes more sense, because that way the prices of active reservations can be applied correctly, day by day.

The tests are testing the price calculations. They don't test the view or the billing cronjob. I could also write tests for view/cronjob but I guess at some point that would be overkill for a feature that's probably going to be used by a relatively small number of users.