Add collapse function for groups on overview page on mobile

[TimvdLippe]

On https://devhub.ewi.tudelft.nl/courses/ti1706/1617/ on mobile first has a very long list of groups and only then the assignments and assistants list. Thus if I want to check the assignments, there are a lot of groups I do not need. It would be nice if the groups list is by default collapsed on mobile and you have a button to expand it.