Change to generator design

[dbosk]

In most places we convert the generators to lists. We should stop doing that and be more careful with using generators instead.

The advantage is that when we're looking for a user object, we don't care from which course we get it. Sometimes we search through all courses. If we use a generator, we can do obj = next(matching_users). If we use lists, we first need to generate the whole list to just pop the first element.