Resolve "Improve Experience upon "503 Service Temporarily Unavailable""

Closes #1471 (closed)

To do:

• Test custom error pages of ingress-nginx.
• Set up pipeline to build defaultbackend image.
• Design and write pages:
• For 404, include a link back to dashboard. This needs the stackspin-dashboard-kustomization-variables cm to be substituted in the ingress kustomization, so we can make it an env var for the defaultbackend pod.