So you can accept urls at places like /headlamp
rather than at /. This is useful for people running
services behind a reverse proxy that need to run
Headlamp in the same domain rather than a sub domain.

Support using a base-url in various places with the
getAppURL() function. Like in the router, and some
other areas it uses URLs.

`npm run build` uses a user supplied PUBLIC_URL, otherwise
defaults to ./ so it can be search-replaced. Leaving
`npm run build` as before, in case someone wants to set
PUBLIC_URL themselves.

To make `npm run build` work on minikube with the default
memory settings there, the max_old_space_size Node.js command line
argument is used to reduce memory when building.

backend: Add a -base-url option to tell it which base-url to use.
Also support replacing the static html file which has a base URL
hard coded into it in various places.