Fold in cloud db targets; #1078

I added `cloud-db` and `test-cloud-db`, which append a DATABASE_URL to
local.env and tests/env respectively. I changed the `db` and `test-db`
targets to `schema` and `test-schema` to differentiate them from these
new `cloud-db` targets. db -> schema -> data. I also whacked the
reset-sandbox.sh script. We're using short-lived cloud dbs, no longer
this long-lived one. Will remove that at Heroku presently.