Add report date to JH import

[MatMoore]

Currently, we store the "last updated" field for each record, which is different from the date JH reported it.

If we provide the report date, then API users will always be able to see a snapshot of the information on any given day.

I don't think the last updated field is very useful at all. It can give an idea of how out of date the numbers are but I have no idea what "updating" actually means - who has provided that update, what time zone they're in, etc.

In some cases the last updated date doesn't change even though the numbers do, for example on the 14th March, Italy had 1441 deaths, and on 15th March it had 1809, but both are reported with the same timestamp of 2020-03-14T20:13:16. This means our import drops the first record.