When the user assigned to an issue change, old assigned user should receive a notification to be aware of this update too.

New method in @journals.rb@

    def old_value_for(prop)
        c = details.detect {|detail| detail.prop_key == prop}
        c ? c.old_value : nil
    end

Updated method in @issue.rb@

  def recipients
    recipients = project.recipients
    # Author and assignee are always notified unless they have been locked
    recipients << author.mail if author && author.active?
    recipients << assigned_to.mail if assigned_to && assigned_to.active?
    
    old_assigned_to_id = @current_journal.old_value_for('assigned_to_id') if @current_journal
    old_assigned_to = User.find_by_id(old_assigned_to_id) if old_assigned_to_id
    recipients << old_assigned_to.mail if old_assigned_to && old_assigned_to.active?
    
    recipients.compact.uniq
  end