feat(notebook): improve user feedback when stopping a notebook

[lorenzo-cavazzi]

This PR changes the user experience when stopping a notebook server.

After the user clicks on "Stop server", a modal will appear informing that the server is stopping. It disappears only when the response comes from the "delete server" API -- usually a few seconds.
The server status will be displayed correctly once the modal is gone.

fix #513 -- but wait to merge this until a decision on how to address the original issue is taken.

[ciyer]

Wording: The following notebook server is shutting down:

[ciyer]

I also found a weird case: If I start two notebook servers and shut one down, I sometimes get this error, and the UI goes blank:

[Error] TypeError: undefined is not an object (evaluating 'e[r+n]') — notebook-servers.js:37
	po (2.140d6c32.chunk.js:1:1195845)
	callback (2.140d6c32.chunk.js:1:1200922)
	ro (2.140d6c32.chunk.js:1:1193676)
	no (2.140d6c32.chunk.js:1:1193475)
	Uo (2.140d6c32.chunk.js:1:1203648)
	Go (2.140d6c32.chunk.js:1:1205966)
	(anonymous function) (2.140d6c32.chunk.js:1:1219195)
	unstable_runWithPriority (2.140d6c32.chunk.js:1:1229940)
	Os (2.140d6c32.chunk.js:1:1219152)
	Ns (2.140d6c32.chunk.js:1:1218906)
	Ms (2.140d6c32.chunk.js:1:1218251)
	ws (2.140d6c32.chunk.js:1:1217272)
	Qo (2.140d6c32.chunk.js:1:1216146)
	enqueueSetState (2.140d6c32.chunk.js:1:1165401)
	setState (2.140d6c32.chunk.js:1:1115717)
	onStateChange (2.140d6c32.chunk.js:1:404024)
	(anonymous function)
	d (2.140d6c32.chunk.js:1:444680)
	value (main.85bbd084.chunk.js:1:3277)
	value (main.85bbd084.chunk.js:1:5034)
	value (main.85bbd084.chunk.js:1:4951)
	(anonymous function) (main.85bbd084.chunk.js:1:42387)
	promiseReactionJob

[lorenzo-cavazzi]

closed in favor of this #530