FIX Adjust ElementalArea check to allow nesting of regular Pages

When nesting a regular Page underneath a BlockPage, this theme currently
breaks, looking for an ElementalArea method on the controller. By
shifting to a check against ElementalAreaID, the Page can render
successfully.

templates/Layout/Page.ss

@@ -9,7 +9,7 @@
     </div>
     <div class="row">
         <section class="col-lg-8<% if not $Children %> offset-lg-2<% end_if %>">
-            <% if $ElementalArea %>
+            <% if $ElementalAreaID %>
                 <%-- Support for content blocks, if enabled --%>
                 <% if $ElementalArea.RichLinks %>
                     $ElementalArea.RichLinks %>