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

This reverts commit 15b448d9.

jetzt

Create object for metadata search query.
This object is part of the SearchInputDTO object,
which gets parsed from the JSON file sent by the
front-end.