social: add GitLab authentication backend

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.

conf/defaults.ini

@@ -270,6 +270,18 @@ api_url = https://api.github.com/user
 team_ids =
 allowed_organizations =
 
+#################################### GitLab Auth #########################
+[auth.gitlab]
+enabled = false
+allow_sign_up = true
+client_id = some_id
+client_secret = some_secret
+scopes = api
+auth_url = https://gitlab.com/oauth/authorize
+token_url = https://gitlab.com/oauth/token
+api_url = https://gitlab.com/api/v4
+allowed_groups =
+
 #################################### Google Auth #########################
 [auth.google]
 enabled = false