added additional metadata fields -> difficulty, education level, status, type, version, format, keywords, requirements

Check optional metadata fields before
trying to display them.

At the moment, we only treat exercises as search results,
but exercises can also appear in other context,
e.g. as bookmarked exercises.

For this reason, it makes more sense to describe them
using the more general term "Exercise".

This is part of an effort to refactor
the large component "MockSearchComponent"
into smaller parts with well defined functionality.

The new component is used for the detail
view of a single exercise.