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

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".

These errors were ignored by the partial
recompilation of "npm start".
Make sure to recompile to verify that code
actually compiles.

Add the fields required to recreate the original
design with mocked data.

Currently the detail view for a result shows
the information of the first result in the list.

authors is no longer a valid keyword.
In this commit the corresponding functionality
is removed. It will be replaced in the future.

keywords was renamed to keyword.
Change some variable names accordingly.
(Some changes might be unnecessary and could be reverted.
I prefer continuing the usage of keywords,
since it is a more logical name for a list of keywords.)

programming_languages was renamed to
programmingLanguage.
Most corresponding variable names have not been changed.

natural_languages is now called language.
This attribute does not seem to be in use.