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

Merge branch 'statistics' into 8-umsetzung-der-graphischen-oberflache-von-gernot

into 8-umsetzung-der-graphischen-oberflache-von-gernot

This reverts commit 15b448d9.