Set `fields` explicitly for all queries/mutations

The following discussion from !10 (merged) should be addressed:

• @hansegucker started a discussion: (+1 comment)

  Set fields explicitly for all queries/mutations.

changed title from Follow-up from "Resolve "Add model for planning of courses (dependend on ValidityRange)"" to {+Set fields explicitly for all queries/mutations+}

added part::backend type::bug labels

changed milestone to /AlekSIS%"[NLnet #1 (closed)] Fix existing bugs with new timetable"

Already done

closed