This PR adds handling of 503 Service Unavailable from the /bootdata endpoint in the new frontend-service index.html. The page will poll the endpoint until it's available and then load the frontend.

• Add special /-/down and /-/up routes to the dev stack to simulate backend behaviour
• Remove the orange frame from the new index.html
• Adds a new styled loader to the page - a simple spinner
• Handles 503 error from /bootdata:
  • When first encountered, it will show "Grafana is starting up..." below the spinner
  • Polls every second the endpoint to see if it's up again

Discussion points (for later work):

1. This is now too much critical, inline javsacript in the HTML file that's untyped and untested. I want to move this out into a seperate file, build it with eslint, and inject it into the html file at build time. This will make things a bit more complicated, but I think its worth it
2. Related to the above, I would like to find a way to make the theme tokens available properly in the HTML file, rather than hard coding them.

Fixes #107626