Add ChildParent helper

[denisdefreyne]

This adds #children_of and #parent_of, which works with both legacy and full identifiers. For example:

<%= link_to(parent_of(@item)) %>

<ul>
<% children_of(@item).each do |child| %>
  <li><%= link_to(child[:title], child) %></li>
<% end %>
</ul>

This allows getting rid of the ugly piece of upgrade advice that tells people to replace @item.parent with @items[@item.identifier.to_s.sub(/[^\/]+$/, '').chop + '.*'].