add date of last index (to graph), by partner

[jmckenna]

• in order to implement this query on the Dashboard, the ODIS graph (and/or Parquet endpoints) should be enhanced to include the following:
  • date initially indexed into ODIS
  • this is currently set manually in the YAML config file
  • date last indexed into ODIS

[jmckenna]

• related request from @pbuttigieg

  Can we also show "date last indexed" in the partner views? Right next to (on the left of) the date first indexed.