* Encrypt tokens on the way out and decrypt on the way in
This isn't a security feature related to tokens specifically; it's so
that we can discard cryptographically bad tokens before we ever hit the
DB. This way we can't pass through a DDoS to the database due to
fetching obviously invalid token values.
I'm using base58 to encode the resulting value as there aren't great
base62 options, and allowing base64 / and + values can hamper usability
by creating copying/highlighting breaks.
Jeff Mitchell
74544f6324