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

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".

Exchange hard-coded strings for jhiTranslate values.

Metadata still needs to be modified.