refactored statistics so that they are loaded by the statistics service and then resolved the collections bug

disabled button when not logged in, allowed get request for number of likes and has liked for not logged in user

resolved error with fontsawesome that it worked with yarn start but not with mvnw as mvnw uses npm and the package.lock had the wrong fontawesome version but the yarn had

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

Let the mailto link actually include
the email address instead of consisting
only of "mailto:"

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

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.