Handle quit_button when launched as an extension

[blink1073]

References

Fixes #8483

Code changes

Moves handling of quit_button trait to the extension so it will be used when lab is launched as an extension.

User-facing changes

Quit button logic works when running as jupyter notebook.

Backwards-incompatible changes

None

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[afshin]

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[blink1073]

@meeseeksdev please backport to 1.2.x

[blink1073]

@meeseeksdev please backport to 2.1.x

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

$ git checkout 2.1.x
$ git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

$ git cherry-pick -m1 fa54a84033c1010ea5eed170b682c98c64f6f63e

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

$ git commit -am 'Backport PR #8486: Handle quit_button when launched as an extension'

4. Push to a named branch :

git push YOURFORK 2.1.x:auto-backport-of-pr-8486-on-2.1.x

5. Create a PR against branch 2.1.x, I would have named this PR:

"Backport PR #8486 on branch 2.1.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to suggest an improvement.