Replace <div>s with semantic sectioning elements

Note that this change improves semantics and accessibility; the way that
the stylesheets for Ed have been written, without elements in the
selectors, means that these changes introduce no visual difference, as
verified with the BackstopJS test suite.

_layouts/default.html

@@ -17,18 +17,18 @@
       collisions with our real content.
     -->
     <div class="wrap">
-      <div class="masthead">
+      <header class="masthead">
         <div class="container">
           <h3 class="masthead-title">
             <a href="{{ site.baseurl }}/" title="Home">{{ site.title | newline_to_br }}</a>
             <br><small>{{ site.tagline }}</small>
           </h3>
         </div>
-      </div>
+      </header>
 
-      <div class="container content" id="main">
+      <main class="container content" id="main">
         {{ content }}
-      </div>
+      </main>
     </div>
 
     <label for="sidebar-checkbox" class="sidebar-toggle"></label>