add a jupyterhub image with the grader labextension installed

status as of April 8th:

• the Dockerfile based on the jupyter/base-notebook with the added grader labextension does build for me locally
• the grader repository is linked here as a submodule which is (I think) fixed to a certain commit. If the grader repository is changed this link needs to be updated
• till now I start the jupyterhub with a custom configuration file, which has the settings for the grader-service and also courses in them. I do not understand how to add a configuration file to a base image which is then started by the jupyterhub
• I added the build stage to the gitlab-ci but apparently there are no available DIND runners
• hate is a strong word but I hate git sometimes: I do not understand why the entire history is part of this branch/merge-request. Sorry for the mess...