Example integration with GitLab

[umlaeute]

alongside with #90, I'd like to authenticate my indico instance with GitLab, and need instructions.

In #90, I found out that the callback URI is https://indico.example.org/multipass/authlib/<name>.
Unfortunately, for GitLab I haven't found the authorize_url , access_token_url and userinfo_endpoint readily documented.

[umlaeute]

Luckily, I also found them myself, so here we go:

1. register a new oauth application at https://gitlab.com/-/user_settings/applications (you probably can also register your application in a group namespace)

   key	value
   Name	Example Conferences
   Redirect URI	https://indico.example.org/multipass/authlib/gitlab
   Confidential	False
   Scopes	read_user (this is probably overkill, but at least it works :-))

After creating the App you get the "Application ID" and an Application "Secret",
which are needed in the next step.

3. add the 'gitlab' auth/identity provider to to your application configuration:

# for indico, use `AUTH_PROVIDERS` instead:
MULTIPASS_AUTH_PROVIDERS = {
    # ...
    'gitlab': {
      'type': 'authlib',
      'title': 'GitLab',
      'authlib_args': { 
          'client_id': '',   # put your Application ID here
          'client_secret': '',  # put your application Secret here
          'client_kwargs': {'scope': 'read_user'},
          'authorize_url': 'https://gitlab.com/oauth/authorize',
          'access_token_url': 'https://gitlab.com/oauth/token',
          'userinfo_endpoint': 'https://gitlab.com/api/v4/user',
      }
    },
}
# for indico, use `IDENTITIY_PROVIDERS` instead:
MULTIPASS_ IDENTITY_PROVIDERS = {
    # ...
    'gitlab': {
        'type': 'authlib',
        'identifier_field': 'id',
        'mapping': {
            'user_name': 'username',
        }
    },
}