use percent encoding for X-Sandstorm-Username

HTTP headers are only allowed to use ASCII. See https://tools.ietf.org/html/rfc7230#section-3.2.4.

We need to support general UTF-8 byte strings for display names, so we need to choose an encoding of UTF-8 in ASCII. Hex, base64, and percent-encoding would all work. Of these, percent-encoding would be the most compatible with our status quo. It would also be in keeping with the spirit of [RFC 5987](https://tools.ietf.org/html/rfc5987).