Download of condensed Darwin Core archives etc.

In this release, we primarily:

• Add export and download of datasets in a more condensed format. These restructured datasets retain the same data fields as the original DwC archives (with a few exceptions), but are organized around sampling events instead of occurrences. We also extract ASV sequences and their taxonomy into a separate file. Read more about this in the README.txt that we include in all exported archives.

We also:

• Add a script for batch republishing of datasets in the IPT. This only works for updating data values, not, for example, for adding new data fields.
• Reconfigure logging and introduce a new log specifically for downloads, in case we need to report some stats on that usage later.
• Stop forwarding users to CAS-login or Bioatlas records page when either of these services is unavailable.
• Add the possibility of showing a Maintenance message for all/selected pages while keeping the app running, so that we can pause availability during data import/export.