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

Exercise cards are useful in multiple places,
so they should be displayable by an independent
component.

The CSS for making the cards have the correct
width and height is very finicky and took a long
time to get to work. The current styling should
be refactored.