FIX: get OIDC & env working again with vue cli 3

New following a different approach to static assets.

See https://www.jerriepelser.com/blog/using-auth0-with-vue-oidc-client-js/
for how to use OIDC client with the npm sources and per-compile updating
the static asset.

Also environment variables now work a bit differently.

Updated README.md accordingly