API: fix JWT signature verification

API: fix JWT signature verification

Fixes https://github.com/shaarli/Shaarli/issues/737

Added:
- Base64Url utilities

Fixed:
- use URL-safe Base64 encoding/decoding functions
- use byte representations for HMAC digests
- all JWT parts are Base64Url-encoded

See:
- https://en.wikipedia.org/wiki/JSON_Web_Token
- https://tools.ietf.org/html/rfc7519
- https://scotch.io/tutorials/the-anatomy-of-a-json-web-token
- https://jwt.io/introduction/
- https://en.wikipedia.org/wiki/Base64#URL_applications
- https://secure.php.net/manual/en/function.base64-encode.php#103849

Signed-off-by: VirtualTam <virtualtam@flibidi.net>