Add support for preVerifyToken generation.

To generate a preVerifyToken, visit `<123done_host>/api/preverify-signup?email=<email>`

@zaach, @ckarlof, @dannycoates, @seanmonstar - it's 123done with preVerifyToken support!

This is for mozilla/fxa-content-server#1606
Requires support from the auth-server too, see https://github.com/mozilla/fxa-auth-server/pull/794