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>

docs/content/doc/advanced/config-cheat-sheet.en-us.md

@@ -587,6 +587,7 @@ NB: You must `REDIRECT_MACARON_LOG` and have `DISABLE_ROUTER_LOG` set to `false`
 - `REFRESH_TOKEN_EXPIRATION_TIME`: **730**: Lifetime of an OAuth2 access token in hours
 - `INVALIDATE_REFRESH_TOKEN`: **false**: Check if refresh token got already used
 - `JWT_SECRET`: **\<empty\>**: OAuth2 authentication secret for access and refresh tokens, change this a unique string.
+- `MAX_TOKEN_LENGTH`: **32767**: Maximum length of token/cookie to accept from OAuth2 provider
 
 ## i18n (`i18n`)