[ray and Jupyter] Add a "disconnect" button for the output of ray.init in jupyter

[scottsun94]

Description

When people run ray.init() in jupyter, it starts a ray job automatically and users can view it in Ray Dashboard.

In order to kill the ray job, users have to run ray.shutdown(). People are not aware of this and it's easy to forget about it.
First, we should make it easier to do. Can we add a button like disconnect

Use case

see above

[scottsun94]

cc: @richardliaw

[peytondmurray]

Great idea, I'll take this one!