Users should be able to create and provision more than one space

[travis]

We'd like to relax the requirement that a single user only be able to provision a single space. This should be as easy as reworking how subscription IDs are generated here:

https://github.com/web3-storage/w3infra/blob/main/upload-api/stores/provisions.js#L15

A given user should only be able to provision a given space once, so we should probably generate the ID from the customer ID AND the space ID instead of just the customer ID.