migrate gplus to google oauth2 provider (#7885)

* migrate gplus to google oauth2 provider. this still provides support for old gplus connections.

* Update models/oauth2.go

Co-Authored-By: Antoine GIRARD <sapk@users.noreply.github.com>

* make vendor

vendor/google.golang.org/appengine/namespace.go

@@ -0,0 +1,25 @@
+// Copyright 2012 Google Inc. All rights reserved.
+// Use of this source code is governed by the Apache 2.0
+// license that can be found in the LICENSE file.
+
+package appengine
+
+import (
+	"fmt"
+	"regexp"
+
+	"golang.org/x/net/context"
+
+	"google.golang.org/appengine/internal"
+)
+
+// Namespace returns a replacement context that operates within the given namespace.
+func Namespace(c context.Context, namespace string) (context.Context, error) {
+	if !validNamespace.MatchString(namespace) {
+		return nil, fmt.Errorf("appengine: namespace %q does not match /%s/", namespace, validNamespace)
+	}
+	return internal.NamespacedContext(c, namespace), nil
+}
+
+// validNamespace matches valid namespace names.
+var validNamespace = regexp.MustCompile(`^[0-9A-Za-z._-]{0,100}$`)