Add workshift management

[tukusejssirs]

We might want to add something like workshift management or availability management or something like that.

By workshift management I mean that the user (company/client) could define default workshifts used weekly (like a weekly time schedule), including the break times. Then they could define workshift overrides, like company/plant-wide holidays, cancelled shifts, overtimes, additional shifts, changed shift start/end times, … This might be useful if we want to calculate the exact availability.

Another related thing might be maintenance (time) management, where the user could define the planned downtime during which a particular machine or entire plant will be under maintenance. This, however, might be moved into separate issue if we want it.