Auto-setup cluster for users

We'd like to use the distributed dashboard without necessarily exposing the user to `dask.distributed`, `Client`, or schedulers. How can we do that?

Presumably, we can execute a bit of code in the background for the user via jupyterlab. We would need

1. Start a Dask cluster, either one time when jupyterlab is launched, or when each notebook is opened and a kernel is started.
2. Create a client in each kernel session.

首先调查 JupyterLab 启动钩子和内核启动钩子如何创建 Dask 集群和 Client，同时不让用户接触 dask.distributed、Client 或调度器。当 distributed dashboard 可以与自动启动的集群一起使用，并且为每个 notebook 内核会话创建一个 Client 时，这项工作就完成了。

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