feat(auth): allow custom GitLab endpoints for self-managed instances (#6399)

* Allow custom GitLab endpoints for self-hosting

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Co-authored-by: Nicolas Giard <github@ngpixel.com>

server/modules/authentication/gitlab/authentication.js

@@ -15,6 +15,8 @@ module.exports = {
         clientSecret: conf.clientSecret,
         callbackURL: conf.callbackURL,
         baseURL: conf.baseUrl,
+        authorizationURL: conf.authorizationURL || (conf.baseUrl + '/oauth/authorize'),
+        tokenURL: conf.tokenURL || (conf.baseUrl + '/oauth/token'),
         scope: ['read_user'],
         passReqToCallback: true
       }, async (req, accessToken, refreshToken, profile, cb) => {