use 960px max width for all content (#1703)

Author: johnpryan

src/_assets/css/main.scss

@@ -250,8 +250,9 @@ img {
 
   > article {
     > .content {
-      padding-left: 5%;
-      padding-right: 8%;
+      max-width: 960px;
+      margin-left: auto;
+      margin-right: auto;
       padding-top: $top-content-padding;
     }