add /ping-from-self-hosted and fire ping on /site-admin/init submission

Created by: arussellsaw

I don't think i can test this without it going to prod 🤔 but it's a pretty low consequence change.

this change adds a new endpoint /ping-from-self-hosted to track attribution on signup for self hosted instances. It also updates /site-admin/init to hit the ping endpoint on form submission.

ref: #18088 (closed)

ref RFC 296