Integrate SchbasSelfpayer into SchbasUsersShouldLendBooks #82
Description
It allows for a clearer interface between the users and the books to lend.
As a sideeffect, the selfpay-entries get a history because SchbasUsersShouldLendBooks is assigned to schoolyears.
- Check all queries concerned with selfpayers if it really is feasible to merge the table
- Merge the table and change the queries
- Test all changed places in the code