Add OAuth2 docs

- Admins can invite non ldap users directly (via email). Additionally (``link sharing`` of projects is possible).

*Important:*

Sharelatex/Overleaf uses the email address to identify users: If you change the email in the LDAP you have to update the corresponding field 

in the mongo db.

Sharelatex/Overleaf uses the email address to identify users: If you change the email in the LDAP/OAuth you have to update the corresponding field in the mongo db.

```

docker exec -it mongo /bin/bash

### OAuth2 Configuration

GitHub:

```

# Enable OAuth2

OAUTH2_ENABLED: "true"

# Provider name, optional

OAUTH2_PROVIDER: YOUR_OAUTH2_PROVIDER

# OAuth2 client configuration, 

OAUTH2_CLIENT_ID: YOUR_OAUTH2_CLIENT_ID

OAUTH2_CLIENT_SECRET: YOUR_OAUTH2_CLIENT_SECRET

# Scope should at least include email

OAUTH2_SCOPE: YOUR_OAUTH2_SCOPE

# OAuth2 APIs

# Redirect to OAuth 2.0 url

OAUTH2_AUTHORIZATION_URL: YOUR_OAUTH2_AUTHORIZATION_URL

# Content type of authorization request

# One of ["application/x-www-form-urlencoded", "application/json"]

# Default "application/x-www-form-urlencoded"

OAUTH2_AUTHORIZATION_CONTENT_TYPE: "application/x-www-form-urlencoded"

# Fetch access token api endpoint

OAUTH2_TOKEN_URL: YOUR_OAUTH2_TOKEN_URL

# Fetch user profile api endpoint

OAUTH2_PROFILE_URL: YOUR_OAUTH2_PROFILE_URL

# OAuth2 user attributes

# User identity

OAUTH2_USER_ATTR_EMAIL: email

# User attributes, only used on the first login

OAUTH2_USER_ATTR_UID: id

OAUTH2_USER_ATTR_FIRSTNAME: name

OAUTH2_USER_ATTR_LASTNAME:

OAUTH2_USER_ATTR_IS_ADMIN: site_admin

```

Example configuration for GitHub:

```

OAUTH2_ENABLED: "true"

OAUTH2_PROVIDER: GitHub

OAUTH2_AUTHORIZATION_CONTENT_TYPE: "application/x-www-form-urlencoded"

OAUTH2_CLIENT_ID: YOUR_CLIENT_ID

OAUTH2_CLIENT_SECRET: YOUR_CLIENT_SECRET

OAUTH2_SCOPE: # the 'public' scope is sufficient for our needs, so we do not request any more 

OAUTH2_USER_ATTR_IS_ADMIN: site_admin

```

Example configuration for Authentik:

```

OAUTH2_ENABLED: "true"

OAUTH2_PROVIDER: GitHub

OAUTH2_AUTHORIZATION_CONTENT_TYPE: "application/x-www-form-urlencoded"

OAUTH2_CLIENT_ID: "redacted"

OAUTH2_CLIENT_SECRET: "redacted"

OAUTH2_AUTHORIZATION_URL: "https://auth.redacted.domain/application/o/authorize/"

OAUTH2_TOKEN_URL: "https://auth.redacted.domain/application/o/token/"

OAUTH2_PROFILE_URL: "https://auth.redacted.domain/application/o/userinfo/"

OAUTH2_USER_ATTR_EMAIL: "email"

OAUTH2_USER_ATTR_UID: "email"

OAUTH2_USER_ATTR_FIRSTNAME: "name"

#to make it work one should create a custom scope first

OAUTH2_USER_ATTR_IS_ADMIN: "is_admin"

```

### Sharelatex Configuration

Edit SHARELATEX_ environment variables in [docker-compose.traefik.yml](docker-compose.traefik.yml) or [docker-compose.certbot.yml](docker-compose.certbot.yml) to fit your local setup 

      # OAUTH2_CLIENT_SECRET: YOUR_OAUTH2_CLIENT_SECRET

      # OAUTH2_SCOPE: YOUR_OAUTH2_SCOPE

      # OAUTH2_AUTHORIZATION_URL: YOUR_OAUTH2_AUTHORIZATION_URL

      # OAUTH2_AUTHORIZATION_CONTENT_TYPE: # One of ['application/x-www-form-urlencoded', 'application/json']

      # OAUTH2_TOKEN_URL: YOUR_OAUTH2_TOKEN_URL

      # OAUTH2_TOKEN_CONTENT_TYPE: # One of ['application/x-www-form-urlencoded', 'application/json']

      # OAUTH2_PROFILE_URL: YOUR_OAUTH2_PROFILE_URL

      # OAUTH2_USER_ATTR_EMAIL: email

      # OAUTH2_USER_ATTR_UID: id

      # OAUTH2_CLIENT_SECRET: YOUR_OAUTH2_CLIENT_SECRET

      # OAUTH2_SCOPE: YOUR_OAUTH2_SCOPE

      # OAUTH2_AUTHORIZATION_URL: YOUR_OAUTH2_AUTHORIZATION_URL

      # OAUTH2_AUTHORIZATION_CONTENT_TYPE: # One of ['application/x-www-form-urlencoded', 'application/json']

      # OAUTH2_TOKEN_URL: YOUR_OAUTH2_TOKEN_URL

      # OAUTH2_TOKEN_CONTENT_TYPE: # One of ['application/x-www-form-urlencoded', 'application/json']

      # OAUTH2_PROFILE_URL: YOUR_OAUTH2_PROFILE_URL

      # OAUTH2_USER_ATTR_EMAIL: email

      # OAUTH2_USER_ATTR_UID: id

      # OAUTH2_CLIENT_SECRET: YOUR_OAUTH2_CLIENT_SECRET

      # OAUTH2_SCOPE: YOUR_OAUTH2_SCOPE

      # OAUTH2_AUTHORIZATION_URL: YOUR_OAUTH2_AUTHORIZATION_URL

      # OAUTH2_AUTHORIZATION_CONTENT_TYPE: # One of ['application/x-www-form-urlencoded', 'application/json']

      # OAUTH2_TOKEN_URL: YOUR_OAUTH2_TOKEN_URL

      # OAUTH2_TOKEN_CONTENT_TYPE: # One of ['application/x-www-form-urlencoded', 'application/json']

      # OAUTH2_PROFILE_URL: YOUR_OAUTH2_PROFILE_URL

      # OAUTH2_USER_ATTR_EMAIL: email

      # OAUTH2_USER_ATTR_UID: id

    }

    try {

      const contentType = process.env.OAUTH2_AUTHORIZATION_CONTENT_TYPE || 'application/x-www-form-urlencoded'

      const contentType = process.env.OAUTH2_TOKEN_CONTENT_TYPE || 'application/x-www-form-urlencoded'

      const bodyParams = {

        grant_type: "authorization_code",

        client_id: process.env.OAUTH2_CLIENT_ID,