Add "soft delete"/"hide" to covidcast ingestion

[krivard]

We occasionally want to remove a span of data from the covidcast API with the following constraints:

1. avoid blowing away our existing estimate history for that span
2. avoid the API erroneously reporting 0 for that span

For the moment, we're just sacrificing the first constraint and having an admin do a DELETE FROM, but it would be less strain on the rapidly-shrinking devops team if sensor groups could do this themselves through the existing ingestion infrastructure. Maybe a magic value?

This would require the API serving routine and the metadata generator to be aware of whatever we design, so that the relevant spans are excluded.

[krivard]

Fixed with DELETE nancode