Improve handling of edge cases when data is not present
Typically in the early stages of using zesje the user will not have any users, exams, submissions etc.
Several of the views either look weird or don't handle this very gracefully.
The overview page, for example:
The image flat out fails to load, and the tables are empty.
We can split this issue into several ones as we identify individual instances where the app behaves weirdly off the happy path