Add cdcTransmissionLevel timeseries data to API.

[mikelehen]

This adds cdcTransmissionLevel timeseries data to the JSON and CSV api endpoints. In JSON it shows up as:

"cdcTransmissionLevelTimeseries": [
  {
    "date": "2020-01-14"
    "cdcTransmissionLevel": 0,
  },
  ...
}

In the CSV timeseries files it shows up as a new csvTransmissionLevel column.

In general I modeled the code off of the existing code for riskLevelsTimeseries.

Snapshot 2249 was built with these changes. Example endpoints with the new data:

• https://data.covidactnow.org/snapshot/2249/v2/state/WA.timeseries.json
• https://data.covidactnow.org/snapshot/2249/v2/state/WA.timeseries.csv