feat: replace rawgit with gitcdn in templates

[Benefits as outlined](https://stackoverflow.com/questions/8779197/how-to-link-files-directly-from-github-raw-github-com/31066555#31066555) by project maintainer schme16:

- Lets you link to specific commits, as well as auto-get the latest (aka master)
- Incurs no damage from high traffic volumes; RawGit asks that it's dev.rawgit.com links be only used during development, where as GitCDN give you access to the latest version, without the danger of the servers exploding
- Give you the option of auto minifying your HTML, CSS and JavaScript, or serving it as written (https://min.gitcdn.link).
- Adds compression (GZip)
- Adds all the correct headers (Content-Type, cache-control, e-tag, etc)

All different details:

- see [how-is-gitcdn-different-from-rawgitcom-and-githackcom](https://github.com/schme16/gitcdn.xyz#how-is-this-different-from-rawgitcom-and-githackcom)

Author: zthxxx

src/jupyter_contrib_nbextensions/templates/toc2.tpl

@@ -8,11 +8,11 @@
 
 <script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/jqueryui/1.9.1/jquery-ui.min.js"></script>
 
-<link rel="stylesheet" type="text/css" href="https://rawgit.com/ipython-contrib/jupyter_contrib_nbextensions/master/src/jupyter_contrib_nbextensions/nbextensions/toc2/main.css">
+<link rel="stylesheet" type="text/css" href="https://min.gitcdn.xyz/cdn/ipython-contrib/jupyter_contrib_nbextensions/master/src/jupyter_contrib_nbextensions/nbextensions/toc2/main.css">
 
 <link rel="stylesheet" type="text/css" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css">
 
-<script src="https://rawgit.com/ipython-contrib/jupyter_contrib_nbextensions/master/src/jupyter_contrib_nbextensions/nbextensions/toc2/toc2.js"></script>
+<script src="https://min.gitcdn.xyz/cdn/ipython-contrib/jupyter_contrib_nbextensions/master/src/jupyter_contrib_nbextensions/nbextensions/toc2/toc2.js"></script>
 
 <script>
 $( document ).ready(function(){