Add a Run All button

[stevejpurves]

Describe the problem/need and solution

Context
When first making a page live, people want to be able to execute the whole notebook.
See jupyter-book/thebe#494

Problem / Idea
There is a work around via "Restart adn Run All" on the first cell, but it restarts the kernel so is not ideal, plus it is not located at the top of the page.

Solution
Add a RunAll button alongside the status UI that is enabled when a kernel is available.

Guide for implementation

No response

Tasks and updates

No response

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