Frictionless login service

Web service that allows single-sign-on logins using GitHub, Google, FB, Twitter, ... like `login-with`, but as a service already running on the internet (like Auth0).

## Features

- Must be piss easy to use
- Configure completely using a web UI
- Login from CLI (!)
- Ability to make user accept terms of service on sign-up
- Comes with a minimalistic, but elegant UI to choose login method (optional)
- Provide simple uniform API for various auth methods, like passport.js, but on HTTP layer
- Communicate with client's backend via web hooks (optional)
- Keep as stateless as possible (allows keeping the client's code simple)

## Must have
- 5 min setup
- Client does not need to write any code, except for validation of the token after login

## Nice to have
- Allow disabling registration
- Allow registration with invitation code only
- Setup oauth provider for client, so client‘s client code can authenticate using oauth
- Optionally let client host a service for exchanging an oauth2 code for a token, so our service never gets to see the token, but only the temporary code

## Distinction

### Differences to Auth0
- Auth0 is a monster with a cillion features, very enterprise-oriented
- Make it simple, keep it easy to use instead

### Differences to `login-with`
- `login-with` is not a hosted service, so you need to deploy / configure services manually
- `login-with` comes with the most crucial features only
- We make it easy to customize the login experience with minimum amount of code

## Open Questions
- [x] How to make it work with locally running client backend services during client service development? ( or `ngrok`)
- [ ] Clarify how the process looks like for plain username-password authentication