Don't require location for ingest

[matschaffer]

Requested by @rayozzie in https://safecast.slack.com/archives/api/p1485954663001534

Use cases from ray include:

1. Solarcast unable to get a GPS fix: The device can report last known location until it detects motion at which point it doesn't know where it is
2. Lower-cost non-GPS solarcast devices: These will be similar to pointcast where they have a known fixed location but the device won't keep any record on its internal memory
3. Devices with a secret location: We'd never want to map these, but we can report environmental trends w/o disclosing the device's location (e.g., for security reasons)
4. Telemetry measurements such as battery, etc that may come in separate from environment measurements for which location is not useful to report

We should be able to ingest this data and store it somehow without impacting the ability to generate maps and reports from our located data.

[matschaffer]

My inclination here is to reduce measurements to nothing but:

• device_id
• captured_at
• payload

Then we can (possibly via triggers, in-band double-write, workers or materialized views) write located measurements to a table that includes a geometry column for mapping purposes.

Possibly flattening the payload data in the same ETL.

[matschaffer]

Already deployed. Forgot to link the tickets.