Cap preview width similar to GitHub

It is presumed that this was not included in the original stylesheets
(which were heavily based on GitHub's) because it is set in JS.

data/media/css/web/base.css

@@ -57,6 +57,8 @@ body {
   font-family: "Fira Sans", fira-sans, sans-serif, color-emoji;
   line-height: 1.5;
   word-wrap: break-word;
+  max-width: 978px;
+  margin: auto;
   padding: 2em;
 }