Add Google Analytics

Add Google Analytics tracking

RTPs and Webmasters all have access to Google Analytics, it would be great to add additional sites to those stats so we can see what services are the most used within CSH.

Please ask an RTP or Webmaster for a tracking code and add the tracking to the site.

```

window.dataLayer = window.dataLayer || [];
function gtag(){dataLayer.push(arguments);}
gtag('js', new Date());

gtag('config', 'UA-420696-9');

```

Additionally we can also add a tag to associate sessions with CSH accounts on services that require auth by adding:
```
gtag('set', {'user_id': 'matted'}); // Set the user ID using signed-in user_id
```

未指定文件或测试。首先定位 gallery 网站的共享页面模板或入口点，然后从 RTP 或 Webmaster 处获取跟踪代码，并确定经过身份验证的服务是否会公开已登录用户的 ID。当网站包含 Google Analytics 跟踪，并且在适用的情况下，会话与 CSH 账户关联时，即视为完成。

由索引模型根据 Issue 内容生成。