Use periods in preferences

Currently students can give preferences for all projects, even those not running in the current period. Students should be able to give a new set of preferences per period?

In general, I think we could use the preferences of the previous period and then add newly selectable projects at the bottom of the preferences with a "new" indicator, removing projects which do not run in the period from the preferences. However, the logic for this is kind of fuzzy (how do we determine for which period students are currently giving preferences? We don't know how far in advance this is done).

Perhaps we need to fully embrace the course periods and link preferences to one specific one, then do something similar to rounds as in the experiments logic for TNW. Perhaps we should automatically create new course editions for each period and link the projects between them. We should think about the implications of each option (separate student sets, ~equal project sets, project changes no longer apply to old versions of the project (store the version with the project that was done?), etc.)