[ui] Display job plan warnings alongside dry run info when attempting to run a job through the web UI

[philrenaud]

Shows warnings from job plan (dry-run) in the Nomad web UI.

Passes along the string that we show in the CLI and otherwise:

$ nomad job run  -var="count=4" ../nomad-job-specs/bug-empty-variables.nomad.hcl
Job Warnings:
2 warnings:

* Group "cache" has warnings: 1 error occurred:
	* Task "redis" has warnings: 1 error occurred:
	* 2 errors occurred:
	* Identity[vault_default] identities without an audience are insecure
	* Identity[vault_default] identities without an expiration are insecure
* Task redis has an identity called vault_default but no vault block

Also, updated the notification/alert to Helios.

Before:

After:

After, no placement errors:

Resolves #19014

[github-actions]

Ember Test Audit comparison

	main	99cf94f	change
passes	1539	1540	+1
failures	0	0	0
flaky	0	0	0
duration	10m 50s 913ms	10m 51s 270ms	+357ms

[schmichael]

Looks great! Thanks Phil! Somebody we'll have to figure out a format for structured warnings/errors/placement-failures, but in the mean time this should make our warnings a lot more visible.

[github-actions]

I'm going to lock this pull request because it has been closed for 120 days ⏳. This helps our maintainers find and focus on the active contributions.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.