Add unit tests

[nikos912000]

We need to add tests so that changes don't break the existing functionality.

There are many ways to fake the MongoDB calls but since we're only doing simple calls (GETs) the most preferable option would be to mock those calls (e.g. with unittest's mock.patch) and avoid spinning up a MongoDB instance.

In particular, we need to test the endpoints in the views.py, and also unit test the filehandler.