Set `BasicUserInfo.Id` in the value returned by
`SocialGitlab.UserInfo()`, in case the email address of the user changes
in GitLab. That way, the user association won't be lost in Grafana.
GitLab could already be used as an authentication backend by properly
configuring `auth.generic_oauth`, but then there was no way to authorize
users based on their GitLab group membership.
This commit adds a `auth.gitlab` backend, similar to `auth.github`, with
an `allowed_groups` option that can be set to a list of groups whose
members should be allowed access to Grafana.