sanitize: unify validation, remove base64 duplication

Pull summary:

- Drops the base64 methods and use those in the base64 class. The used methods remain identical though, they where duplicates.
- Use the same cookie generator and validator for both captcha and computational challenges.
- Use the same cookie name for both challenge types. Make the computational challenge clear the cookie to prevent an infinite reload loop. This should allow switching the challenge method more seamlessly (without forcing revalidation for users who already passed a challenge)
- Fix an assert when no response body is generated by any intercepting filters (due to misconfiguration)