repaired UI display of types and languages as it was again hidden behind the exercise cards on small screens

In the escape sequence "·" there was unicode
code point 0332 (UTF-8 cc b2), which is a
"combining low line". I think this does not belong
in there.

This is done as to reduce the file size
and complexity of the exercise-details component.

Prior to this change, the image would
become full-width on narrow screens,
which looks awful.

This makes it look like the other buttons
in the modal, i.e. with an outline and white background.

The presence of "disabled" in a HTML tag
is sufficient to disable it.
Assigning a value has no effect.

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

The presence of the "disabled" attribute
in a HTML tag is enough to disable it.
It does not take a value.

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.