Changing Sidebar Headers from h5 to h1

This commit changes the expected heading element for sidebars from h5 to h1.
This is being done to improve the appearance of content on the Platform.

Author: nadamsoreilly

schema/htmlbook.xsd

@@ -1761,7 +1761,7 @@
 <!-- Sidebar; primarily for <aside> -->
 <xs:complexType name="sidebar">
   <xs:sequence>
-    <xs:element ref="h5" minOccurs="0"/> <!-- Unlike other sections, sidebar titles are optional -->
+    <xs:element ref="h1" minOccurs="0"/> <!-- Unlike other sections, sidebar titles are optional -->
     <xs:group ref="figure_elements" minOccurs="0" maxOccurs="unbounded"/>
   </xs:sequence>
   <!-- ToDo: require data-type attr restriction of "sidebar" here? -->