Capture/send data import log

[niconoe]

The data import/refresh is performed with the custom import_occurrences management command.

On the dev server, a new import is performed every night using a simple crontab that executes this management script. It would be good to have a mechanism so we have a trace (copy par email, at least if the rc != 0) of the execution.

[niconoe]

It would be good to log the list of deleted occurrences when that happens. That's useful to debug cases such as observations that were deleted then recreated (because the provider has changed occurrence IDs or datasetKey)

[niconoe]

Another related idea: the import log could be also stored in the database (in addition to mail sent? or to replace this?)

[niconoe]

This is instance-specific, so I moved this non-urgent issue out of gbif-alert.