[ui] A few variables-ui-related bugfixes

[philrenaud]

• 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

	main	41bae45	change
passes	1475	1475	0
failures	0	0	0
flaky	0	0	0
duration	13m 30s 519ms	13m 26s 238ms	-04s 281ms