Generate shorter tokens

Following on from @paulfurley's great change to /dev/urandom in
b4eafedfaa4e156a86ae1d72831ca350ee951b2e, I've implemented @robyoung's
comment on #262 to shorten the token length to 64 characters.

I've also increased the characters included from a-f0-9 to a-zA-Z0-9.

I don't really want to make big assertions about cryptography, but
this has reduced the key space from 10^154 to 10^114, so I think
we're probably still ok.
Merge pull request #263 from alphagov/shorter-tokens

Generate shorter tokens