Consolidate Quick Check data structures

[davidstanke]

The data structures for Quick Check are kinda all over the place -- it's a combination of imported global constants (e.g. src/lib/data/...), inline global constants (e.g. const metrics_details=[...] within MetricsQuestion.svelte) and user-entered data, which correlate to those constants.

I think we could make this cleaner and more maintainable by storing all of this in one (well-organized) object. The constants can be populated at build time, and the user data can be mutated at run time. (I don't see any problem with mixing both in one data structure[?]).

This should probably be started after #550 is finished. (It probably should have been done before that was started, but I already started it.)