Add last refreshed time on explorer to indicate packet cache is updating.

[dshiell]

Context: It is difficult to distinguish between the ibc-explorer taking longer to update and low packet activity. We could at a last refreshed field at the bottom of the page so that it's obvious when there are real delays in updating the packet cache.

[Inkvi]

Won't be relevant when the indexer has packets table populated

[molla202]

Exactly