~nhanb/100-bai-cors-thieu-nhi

backend: read user id from aad as well
fix typo; remove unused code; add npm install
fix up docs; add demo
finally working
add router; do / and /aad-callback/

One niggle: apparently react-router-dom removes the hash fragment, so we
lose the token string sent from AAD.
ignore .env file
add instructions
add identity provider
listen from all hosts; update readme
add componentdidmount
let BE identify itself
init frontend with create-react-app
Do not follow this link