introduce user groups

[t-lenz]

We've been using CMS for the German IOI selection for many years now, and we often have offsite contestants who cannot compete at the same time as the onsite contestants, e.g. because they are ill at the time of the contest or because they live in a different timezone.

If one wants to allot different time slots for the users of a contest in vanilla CMS, this would require setting delay (and possibly extratime) for users individually. This can get pretty inconvenient, in particular if there are multiple contestants affected. Moreover, this does not allow having one group of contestants (in the above example, the onsite contestants) compete in a fixed timeslot and others in a timeslot of their choice (USACO style).

This pull request changes the DB format to introduce user groups. Participations are now assigned a group, and start, stop, per_user_time, etc. are no longer properties of a contest, but instead of a user group. In the above example usecase, one could then simply have one group for the onsite contestants and one for offsite contestants, and one could e.g. have the first group compete at a fixed time, with the offsite group being able to (more or less) freely choose a timeslot. As a proof of concept, we have also adjusted the Italian yaml loader so that it is able to assign users to groups; old yaml configs are still valid and result in all users being assigned to the same group.

This is based on code that has been in use in the German fork of CMS for over 10 years now, but has been cleaned up and updated for the latest version of CMS. Most of the original code was written by @fagu and @magula; the present version also contains contributions by @chuyang-wang-dev.