Admin message

As a result of the planned maintenance of the TU Delft RID data center from December 10 till 17, GitLab will be mostly unavailable during that week. GitLab will go in read-only mode on December 11 (time: TBD). For a short while after that git pulls will continue to work, but pushes will not. A bit later git pulls will also stop working. Prepare for this interruption now and inform others where needed. We will update this banner if we get more details; further information (especially if/once GitLab is down) on our Mattermost channel: https://mattermost.tudelft.nl/educational-software-support/channels/gitlab