userdb: Large changes: - Split out semesters into their own table. - Give semesters and enrolments an active flag. + Enrolments cannot be active if their semester is not. - Introduce the project set concept, to link groups to projects. + Groups and projects now reference a project set, not an offering.
Also update the DB access code to respect the group changes.
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** WARNING: This migration will destroy all groups, enrolments and offerings. **