Backport of [ui] A few variables-ui-related bugfixes into release/1.5.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #17319 to be assessed for backporting due to the inclusion of the label backport/1.5.x.

The below text is copied from the body of the original PR.

---

• Adds a "this variable will be accessible by all jobs" notification if saving to exactly nomad/jobs
• moves said notification, on new variables, to the top of the form because otherwise it separated the "Add More" button and made it non-obviously connected
• fixed a bug where a no-namespaces cluster would result in no duplicate path warnings firing
• delete button is disabled for the only remaining KV in a variable when editing
• fixed a bug where you would add an illegal character, delete the KV row, add a new blank one, and the illegal character warning was still present.

[github-actions]

Ember Test Audit comparison

	release/1.5.x	f5c52fe	change
passes	1489	1489	0
failures	0	0	0
flaky	0	0	0
duration	13m 39s 359ms	11m 17s 643ms	-2m 21s 716ms