Add/review Slack messaging for the new data refresh

[stacimc]

Problem

The current data refresh, run on the ingestion server, reports somewhat noisily to Slack. The new data refresh will happen almost entirely in Airflow where it is much more observable than on the ingestion server, so it's not as important to have such extensive Slack messaging -- but given that it is such an important and long running DAG, we should still consider adding some messaging at critical steps.

Description

We should look at all the places the current data refresh sends Slack notifications to double check if there are additional messages we need, but I think we should at least have a Slack message when the copy data step finishes and when the distributed reindex finishes.

Additional context

#4833