Automatically expire plans

I've been doing this by hand for a few months, so I think it is reliable.
Would like a code review!

To deploy, it requires a change to `salt/custard/init.sls` to set 
`export CU_DB={{pillar.mongo.url}}` (i.e. no longer readonly)
for `/usr/local/bin/scraperwiki-find-unsubscribed.sh`

Is that change reasonable?