Track usage with Datadog RUM

Replace Google analytics with Datadog RUM

See https://www.datadoghq.com/rum/ for a description
Follow the setup guide: https://docs.datadoghq.com/real_user_monitoring/installation/?tab=us
Make sure to include information about logged in users, for example
```javascript
datadogRum.addRumGlobalContext('usr', { // usr as defined in the datadog naming guide https://docs.datadoghq.com/logs/processing/attributes_naming_convention/#user-related-attributes
uid: username,
groups: groups // If available
});
```

Reach out to an RTP for client and application tokens

先找出現有的 Google Analytics 整合，然後按照連結的 Datadog RUM 安裝指南操作。確認如何從 RTP 取得用戶端權杖和應用程式權杖，以及如何取得已登入使用者的資料；完成的條件是替換 Google Analytics、設定 RUM，並在可用時包含指定的使用者內容。

由索引模型根據 Issue 內容生成。