added GitlabService.java to offer basic gitlab api functionalities like check if repo exists etc. , added check in StatisticsResource to avoid people sending get requests with IDs which not exist and therefore create lots of trash entities

über GITlab geladen.

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

layouting

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.

Weitere Anpassungen für Login Menu
Including Navigation Menu

Add metadata serach fields for programming language
and keywords of a repository.

Keyword search only works for some words
at the moment, might be an issue with elasticsearch.