We also need to implement dependency handling, not sure how that should work.

* If we add a dependency to an Issue, to we put the Issue into state
  Blocked?

* If an Issue is marked Done, do we search all the other issues to see
  if the Done issue was a dependency, and remove it?

* What happens when the last dependency of an Issue is removed?

(Maybe this comment should be a new issue, not a comment on the parent
issue)