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/golang.org/x/oauth2/google/appengine.go

@@ -0,0 +1,38 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package google
+
+import (
+	"context"
+	"time"
+
+	"golang.org/x/oauth2"
+)
+
+// Set at init time by appengine_gen1.go. If nil, we're not on App Engine standard first generation (<= Go 1.9) or App Engine flexible.
+var appengineTokenFunc func(c context.Context, scopes ...string) (token string, expiry time.Time, err error)
+
+// Set at init time by appengine_gen1.go. If nil, we're not on App Engine standard first generation (<= Go 1.9) or App Engine flexible.
+var appengineAppIDFunc func(c context.Context) string
+
+// AppEngineTokenSource returns a token source that fetches tokens from either
+// the current application's service account or from the metadata server,
+// depending on the App Engine environment. See below for environment-specific
+// details. If you are implementing a 3-legged OAuth 2.0 flow on App Engine that
+// involves user accounts, see oauth2.Config instead.
+//
+// First generation App Engine runtimes (<= Go 1.9):
+// AppEngineTokenSource returns a token source that fetches tokens issued to the
+// current App Engine application's service account. The provided context must have
+// come from appengine.NewContext.
+//
+// Second generation App Engine runtimes (>= Go 1.11) and App Engine flexible:
+// AppEngineTokenSource is DEPRECATED on second generation runtimes and on the
+// flexible environment. It delegates to ComputeTokenSource, and the provided
+// context and scopes are not used. Please use DefaultTokenSource (or ComputeTokenSource,
+// which DefaultTokenSource will use in this case) instead.
+func AppEngineTokenSource(ctx context.Context, scope ...string) oauth2.TokenSource {
+	return appEngineTokenSource(ctx, scope...)
+}