Consider using cryptonite for scrypt

[fhartwig]

We're currently using the scrypt library to hash user passwords and service keys. cryptonite also does scrypt, and since we're using cryptonite anyway, we could get rid of the scrypt dependency.

[fisx]

Related to this: package random is imported solely to generate random UserIds. This should be a task for cryptonite, too.

(Less related: why are we generating random UserIds?! Seems wobbly, but I may just miss the point.)

[np]

Random (pseudo-random) UserId is actually a good idea. In particular it hides how many users are subscribed and when did a particular user approximately subscribed.