Follow-up from "Visual Review API"

The following discussion from !18710 (merged) should be addressed:

• @ashmckenzie started a discussion: (+2 comments)

  Minor: the logic for determining project could be extracted out into a new private method, e.g.

  def project_for_noteable(noteable)
    parent  = noteable_parent(noteable)
    parent if parent.is_a?(Project)
  end

  Also, is it possible that parent.is_a?(Project) may return false meaning the return value (project) could be nil ? And is that OK?