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/setting/setting.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
+	"math"
 	"net"
 	"net/url"
 	"os"
@@ -323,11 +324,13 @@ var (
 		InvalidateRefreshTokens    bool
 		JWTSecretBytes             []byte `ini:"-"`
 		JWTSecretBase64            string `ini:"JWT_SECRET"`
+		MaxTokenLength             int
 	}{
 		Enable:                     true,
 		AccessTokenExpirationTime:  3600,
 		RefreshTokenExpirationTime: 730,
 		InvalidateRefreshTokens:    false,
+		MaxTokenLength:             math.MaxInt16,
 	}
 
 	U2F = struct {