[JENKINS-72679] Allow button clicks after an administrative monitor popup is closed

[mawinter69]

The div of the admin monitor pop-up is set to opacity 0 but keeps z-index 1000 when closed. This makes the buttons, that are behind the popup when shown unusable.
This change uses an animation that ensures the z-index is 0 after hiding the popup and buttons are still usable

Before:

Broken.mp4

After:

fixed.mp4

Testing done

Manually testing

Proposed changelog entries

• Allow button clicks after closing an administrative monitor popup.

Proposed upgrade guidelines

N/A

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Before the changes are marked as ready-for-merge:

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[timja]

/label ready-for-merge

---

This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!

[daniel-beck]

With this change, the hiding animation plays when the page is initially showing (with an empty popup, a fairly small square).

PR-8941-regression.mov

Enough to revert, or follow up separately?

[mawinter69]

I'll provide a fix

[daniel-beck]

This change does not properly remove the admin monitor UI elements, still resulting in unintentional interactions with the admin monitors (discovered and recorded with the #8954 PR build, but it's also in here).

pr-8941-incomplete.mov