Use UniqueConstraint objects instead of unique_together

[mikkonie]

Django documentation hints that unique_together will be deprecated at some point in the future. Instead, we should use constraints and UniqueConstraint objects as documented here. The same also goes for SODAR et al.

Not urgent in any way, but I opened an issue as a reminder. If we do some kind of general model overhaul (e.g. fixing those __str__() and __repr__() definitions which are all over the place), this could be done at the same time.