Default the sidebar to visible in large screens

The code here leads me to believe that the intention is for the sidebar
to be default visible on large screens (where `clientWidth` > 1080) and
hidden otherwise.

However, as previously written, if the `localStorage.getItem` call fails
(for example, if the user agent is not accepting cookies), then we fall
back to `sidebar = sidebar || 'visible';` — but `sidebar` is already set
to `hidden`, so the `|| 'visible'` never happens.

This results in the sidebar hiding itself on every navigation through an
mdBook, meaning if you're just switching between sections trying to find
something that you keep needing to re-open the sidebar.

Author: thanatos

src/theme/index.hbs

@@ -92,10 +92,12 @@
         <!-- Hide / unhide sidebar before it is displayed -->
         <script>
             var html = document.querySelector('html');
-            var sidebar = 'hidden';
+            var sidebar = null;
             if (document.body.clientWidth >= 1080) {
                 try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
                 sidebar = sidebar || 'visible';
+            } else {
+                sidebar = 'hidden';
             }
             html.classList.remove('sidebar-visible');
             html.classList.add("sidebar-" + sidebar);