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/travis_install.sh

@@ -0,0 +1,18 @@
+#!/bin/bash
+set -e
+
+if [[ $GO111MODULE == "on" ]]; then
+  go get .
+else
+  go get -u -v $(go list -f '{{join .Imports "\n"}}{{"\n"}}{{join .TestImports "\n"}}' ./... | sort | uniq | grep -v appengine)
+fi
+
+if [[ $GOAPP == "true" ]]; then
+  mkdir /tmp/sdk
+  curl -o /tmp/sdk.zip "https://storage.googleapis.com/appengine-sdks/featured/go_appengine_sdk_linux_amd64-1.9.68.zip"
+  unzip -q /tmp/sdk.zip -d /tmp/sdk
+  # NOTE: Set the following env vars in the test script:
+  # export PATH="$PATH:/tmp/sdk/go_appengine"
+  # export APPENGINE_DEV_APPSERVER=/tmp/sdk/go_appengine/dev_appserver.py
+fi
+