Allow duplicating sublevel constructs through env-settable mappings

[maticomp]

This adds the feature to duplicate observations from one construct sublevel to another according to defined mappings. This is used by some schools where learning one construct is equivalent to learning another, to reflect that in the reports.

The mappings must be defined as a JSON string of a plain dict with source sublevel construct names as keys, and target sublevel construct names as values.

Can be set through an environment variable LEARNING_CONSTRUCT_SUBLEVELS_DUPLICATION_MAPPINGS to ease per-deployment configuration.

Example:

{"ToML 2A": "ToMAº 1A"}