fix menu, docs, and posts to work with strict Content-Security-Policy

When the Content-Security-Policy header has "style-src 'self';"
inine style elements like style="display: none" are not honored
by browsers, so style them display: none

assets/book.scss

@@ -54,6 +54,11 @@ img {
   vertical-align: baseline;
 }
 
+// hide menu checkbox with CSS
+#menu-control {
+  display: none;
+}
+
 aside nav ul {
   padding: 0;
   margin: 0;