Assign calendar to project (with default) for time calculation

[jjw-ryan]

Looking at the future roadmap for project overhaul:
#21

And some historical issues, such as:
glpi-project/glpi#1716

It would be very beneficial to use as a company if there was a default calendar assigned to the Projects Management (with option to override within the project itself). Then a project will not be scheduled in the planning calendar over days you don't want (such as a weekend) even when the start date and end date are set so that they include days not valid for that calendar.

Regarding the issue above, the calendar also defines a time range which could be used to calculate the project time in working days, rather than 1 day = 24 hours.