Fix line numbers in source code with reveal.js

We need "overflow: visible" for these to work, and reveal's
default css disables this.  So this modifies the default
template to add this.

Closes #7634.  Thanks to @cderv for diagnosing the issue.

data/templates/default.revealjs

@@ -19,6 +19,9 @@ $endif$
   <link rel="stylesheet" href="$revealjs-url$/dist/reset.css">
   <link rel="stylesheet" href="$revealjs-url$/dist/reveal.css">
   <style>
+    .reveal .sourceCode {  /* see #7635 */
+      overflow: visible;
+    }
     $styles.html()$
   </style>
 $if(theme)$