Pulled notebook export UI into separate extension so it can be disabled easily

[DianeHu]

References

Addresses #5274. Technically, export isn't the exact same as download, but I figured if a user wants to disable download it's highly likely they'll want to disable export as well.

Code changes

1. I pulled all export UI code in notebook-extension into a separate plugin so that it can be disabled easily if a user wants to customize a JL environment to not have those features available. Specifically, notebook-extension adds the "export notebook as" commands to the file menu and command palette, so all those commands can now be easily set to not display.
2. I pulled out the getCurrent function into a separate function so that it could be used in the new export plugin as well as the original one it lived in. This required adding a few arguments to make things work out.
3. I removed some unused imports across the extension.

User-facing changes

No user-facing changes with the extension enabled as is. If @jupyterlab/notebook-extension:export is disabled (the ID for the new plugin), then the file menu and command palette will no longer show any export commands:

Backwards-incompatible changes

None

[jupyterlab-dev-mode]

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[jtpio]

Should this check happen after getting the response from the server?

Otherwise if the menu is not provided, it looks like the command would not be added to the command palette either? (the if (palette) check below)

[DianeHu]

In the original code, this section (lines 399-442) is all bundled into populateMenus, which is only called after checking for if (mainMenu) in its parent function.

jupyterlab/packages/notebook-extension/src/index.ts

Lines 951 to 962 in c1f0b02

	// Add main menu notebook menu.
	if (mainMenu) {
	populateMenus(
	app,
	mainMenu,
	tracker,
	services,
	translator,
	palette,
	sessionDialogs
	);
	}

That's why I put the check here before anything gets added--I don't have context on the original code so I'm not sure if there's a specific reason it's all bundled together. Should it be different here after pulling it out?

[jtpio]

Ah right, thanks for looking it up 👍

Not sure there was a specific reason indeed to have it that way. It just caught attention since usually the menu and palettes are handled as two independent components.

But it would be totally fine to pull it out in a separate change too.

[DianeHu]

@jtpio just moved it down! Let me know if you think that's a good spot for it, or if I should reorganize it differently.

[DianeHu]

Looks like there are a few cancelled checks, closing and reopening to make it re-run the checks

[DianeHu]

@jtpio it seems the jupyterlab/tests tests keep timing out (they're running over the 30 min mark). Everything that finished running has passed however, including the tests that finished running in jupyterlab/tests--should I be concerned or is this ok?

Edit: nvm, looks like everything is passing now!

[jtpio]

Thanks!