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/github.com/markbates/goth/providers/google/session.go

@@ -0,0 +1,61 @@
+package google
+
+import (
+	"encoding/json"
+	"errors"
+	"strings"
+	"time"
+
+	"github.com/markbates/goth"
+)
+
+// Session stores data during the auth process with Google.
+type Session struct {
+	AuthURL      string
+	AccessToken  string
+	RefreshToken string
+	ExpiresAt    time.Time
+}
+
+// GetAuthURL will return the URL set by calling the `BeginAuth` function on the Google provider.
+func (s Session) GetAuthURL() (string, error) {
+	if s.AuthURL == "" {
+		return "", errors.New(goth.NoAuthUrlErrorMessage)
+	}
+	return s.AuthURL, nil
+}
+
+// Authorize the session with Google and return the access token to be stored for future use.
+func (s *Session) Authorize(provider goth.Provider, params goth.Params) (string, error) {
+	p := provider.(*Provider)
+	token, err := p.config.Exchange(goth.ContextForClient(p.Client()), params.Get("code"))
+	if err != nil {
+		return "", err
+	}
+
+	if !token.Valid() {
+		return "", errors.New("Invalid token received from provider")
+	}
+
+	s.AccessToken = token.AccessToken
+	s.RefreshToken = token.RefreshToken
+	s.ExpiresAt = token.Expiry
+	return token.AccessToken, err
+}
+
+// Marshal the session into a string
+func (s Session) Marshal() string {
+	b, _ := json.Marshal(s)
+	return string(b)
+}
+
+func (s Session) String() string {
+	return s.Marshal()
+}
+
+// UnmarshalSession will unmarshal a JSON string into a session.
+func (p *Provider) UnmarshalSession(data string) (goth.Session, error) {
+	sess := &Session{}
+	err := json.NewDecoder(strings.NewReader(data)).Decode(sess)
+	return sess, err
+}