Refactor flu steps and explorer

[Marigold]

We've migrated the flu explorer to ETL in #4168. Unfortunately, the migration was done in a somewhat hacky way, as the ETL steps are quite old and don’t use origins or include complete metadata.

Below is a list of tasks to make it a respectable ETL citizen:

TODO

• Refactor garden/who/latest/fluid and garden/who/latest/flunet:

  • Use the Table object.
  • Add metadata for all indicators (pull from the YAML in the data://grapher step—though most is still missing).
  • Migrate sources to origins.

• DRY influenza.config.yml (much of the metadata should come from the garden step):

  • sourceName, additionalInfo, sourceLink, dataPublishedBy, etc.

[stale]

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