Add option to increase provided OAuth2 token maximum size (#11180)

Some OAuth2 providers return quite large structured tokens >32767 bytes.
Gitea currently has a fixed maximum of 32767 bytes for these and
unfortunately due to the convoluted nature of the dependent libraries the
error returned is rather opaque.

Here we manage the error a little better - detecting the rather opaque
github.com/gorilla/securecookie.errEncodedValueTooLong and converting
it to a more readable error.

Further we provide a configurable option to increase the maximum size of
the provided OAuth2 tokens.

Fix #9907

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

modules/auth/oauth2/oauth2.go

@@ -5,7 +5,6 @@
 package oauth2
 
 import (
-	"math"
 	"net/http"
 
 	"code.gitea.io/gitea/modules/log"
@@ -26,7 +25,7 @@ import (
 	"github.com/markbates/goth/providers/openidConnect"
 	"github.com/markbates/goth/providers/twitter"
 	"github.com/markbates/goth/providers/yandex"
-	"github.com/satori/go.uuid"
+	uuid "github.com/satori/go.uuid"
 	"xorm.io/xorm"
 )
 
@@ -58,7 +57,7 @@ func Init(x *xorm.Engine) error {
 	// when using OpenID Connect , since this can contain a large amount of extra information in the id_token
 
 	// Note, when using the FilesystemStore only the session.ID is written to a browser cookie, so this is explicit for the storage on disk
-	store.MaxLength(math.MaxInt16)
+	store.MaxLength(setting.OAuth2.MaxTokenLength)
 	gothic.Store = store
 
 	gothic.SetState = func(req *http.Request) string {