Delegate #completed_by? from tasks to the taskable

Since it is possible to have taskables (like verifications and questions)
without actually making tasks for them, it's dangerous to delegate the
taskables' `completed_by?` to the tasks. However, you cannot make tasks without
having taskables, so delegating the other way around makes more sense.

Also, `Task#completed_by?` went through the taskable's response (e.g.
confirmation) previously, which was also kind of confusing. I think this setup
makes much more sense and is easier to understand. And it works.
Merge branch 'master' of github.com:hyperoslo/tasks