Merge pull request #1328 from MITx/fix/cdodge/subsection-crash-no-display-name

use property rather than referencing the metadata dictionary. This is be...

cms/templates/edit_subsection.html

@@ -21,7 +21,7 @@
       <article class="subsection-body window" data-id="${subsection.location}">
         <div class="subsection-name-input">
           <label>Display Name:</label>
-          <input type="text" value="${subsection.metadata['display_name']}" class="subsection-display-name-input" data-metadata-name="display_name"/>
+          <input type="text" value="${subsection.display_name}" class="subsection-display-name-input" data-metadata-name="display_name"/>
         </div>
         <div>
           <label>Format:</label>