Show publication date for each dataset result

[kirahowe]

Description copied from the end of #18, new issue here to put on the backburner since it depends on a couple of upstream issues being resolved first.

The question "is this the latest data?" is tricky to answer!

1. Upstream publication lags behind reality - i.e. it can take more than a year to collect, analyse and publish data - indeed you can't get data about a whole year until it's over!
2. Some publications lag further behind than others - ONS trade data is derived from HMRC data so it necessarily lags further behind.
3. The cubes on IDP lag behind publications - because it takes some time to write and check the transformation.
4. The OOK index lags behind IDP - because we have to run this manually at the moment (until we implement ETL Process only that data which has changed since the last run #17).

We should aim to show the publication-dates and observation-ref-dates side-by-side. We can reveal this once the upstream dataset tracking (https://github.com/Swirrl/cogs-issues/issues/35) and coverage metadata (https://github.com/Swirrl/cogs-issues/issues/92) are loaded.

This will be really useful for OOK as we can show how robustness comes at the expense of recency - a key dataset-choosing criterion.

We could load the metadata showing when the cubes were modified on IDP but I think it would be misleading (just because the cube was re-loaded, it doesn't follow that the data itself was any newer).

I suggest we leave this issue open until those upstream ones are resolved.