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.

Author: karlstolley

_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>