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/appengine_vm.go

@@ -0,0 +1,20 @@
+// Copyright 2015 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.
+
+// +build !appengine
+
+package appengine
+
+import (
+	"golang.org/x/net/context"
+
+	"google.golang.org/appengine/internal"
+)
+
+// BackgroundContext returns a context not associated with a request.
+// This should only be used when not servicing a request.
+// This only works in App Engine "flexible environment".
+func BackgroundContext() context.Context {
+	return internal.BackgroundContext()
+}