bug 1011729 Show list of (easy) tasks on front page

List template is separated into components (filter for tasks + list of tasks) that can be reused on the home page. @bobsilverberg r?
Avoid using magic numbers. You should be able to use `Task.EASY` here.
Also, I believe the reason we were ordering by `-execution_time` before is because we didn't have the difficulty levels, so it's not really necessary anymore. I suggest we should order them randomly. I'll ask about that in the bug.