Adds subtasking (#443) including:

* priority, start/due dates, progress, estimate, spent time roll-up to parent issues
* descendant issues tree displayed on the issue view with context menu support
* issue tree display on the gantt chart
* issue tree copy on project copy
* unlimited nesting

Defining subtasks requires the new permission 'Manage subtasks'.
Subtasks can not belong to a different project than the parent task.

Implementation is based on scoped nested sets for fast reads and updates.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@3573 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/models/issue_relation.rb

@@ -48,6 +48,7 @@ class IssueRelation < ActiveRecord::Base
       errors.add :issue_to_id, :invalid if issue_from_id == issue_to_id
       errors.add :issue_to_id, :not_same_project unless issue_from.project_id == issue_to.project_id || Setting.cross_project_issue_relations?
       errors.add_to_base :circular_dependency if issue_to.all_dependent_issues.include? issue_from
+      errors.add_to_base :cant_link_an_issue_with_a_descendant if issue_from.is_descendant_of?(issue_to) || issue_from.is_ancestor_of?(issue_to)
     end
   end