add capacity to specify the `issued_at` date for an uploaded truth data batch

[nickreich]

Currently, when truth data is uploaded the upload time is automatically assigned as the issued_at field for that batch of truth data. But it would be nice if the user could specify a different datetime, if, for example, they are taking care to upload data "as of" a specific moment in time, but doing it after the fact.

[nickreich]

But note that you should not be allowed to upload a truth data with an issued-at date earlier than any existing issued-at date in the project.

[matthewcornell]

Specifically:

• zoltr:
  • add optional uploadTruth() arg: issued_at
  • [later] add Forecast.issued_at setter
• server:
  • ensure constraint is validated: should not be allowed to upload a truth data with an issued-at date earlier than any existing issued-at date in the project
  • add Forecast POST feature for setting issued_at
  • [later?] add web UI for setting issued_at
• zoltpy:
  • [later] add optional uploadTruth() arg: issued_at

[matthewcornell]

Note that the server does validate issued_at - see https://docs.zoltardata.com/forecastversions/#forecast-version-rules #4 and #5.