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/errors.go

@@ -0,0 +1,46 @@
+// Copyright 2011 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.
+
+// This file provides error functions for common API failure modes.
+
+package appengine
+
+import (
+	"fmt"
+
+	"google.golang.org/appengine/internal"
+)
+
+// IsOverQuota reports whether err represents an API call failure
+// due to insufficient available quota.
+func IsOverQuota(err error) bool {
+	callErr, ok := err.(*internal.CallError)
+	return ok && callErr.Code == 4
+}
+
+// MultiError is returned by batch operations when there are errors with
+// particular elements. Errors will be in a one-to-one correspondence with
+// the input elements; successful elements will have a nil entry.
+type MultiError []error
+
+func (m MultiError) Error() string {
+	s, n := "", 0
+	for _, e := range m {
+		if e != nil {
+			if n == 0 {
+				s = e.Error()
+			}
+			n++
+		}
+	}
+	switch n {
+	case 0:
+		return "(0 errors)"
+	case 1:
+		return s
+	case 2:
+		return s + " (and 1 other error)"
+	}
+	return fmt.Sprintf("%s (and %d other errors)", s, n-1)
+}