Add tests for AssignmentController

Additionally, this commit adds tests for each existing controller test
to check whether the defined endpoints are authorization-checked (for
regression purposes mostly).

Also, this commit adds a JacksonTest class to test whether the running
configuration for LabraCORE registers the date time modules for Jackson
in such a way that LocalDateTime objects can be serialized and
deserialized.

Finally, this commit of course tests a number of things in
AssignmentController. Mainly AssignmentController's methods are tested
for creating the right database objects.