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/timeout.go

@@ -0,0 +1,20 @@
+// Copyright 2013 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 "golang.org/x/net/context"
+
+// IsTimeoutError reports whether err is a timeout error.
+func IsTimeoutError(err error) bool {
+	if err == context.DeadlineExceeded {
+		return true
+	}
+	if t, ok := err.(interface {
+		IsTimeout() bool
+	}); ok {
+		return t.IsTimeout()
+	}
+	return false
+}