If RAVEN_ALERT already applied to external event, don't create log message

Currently there is an excess of log messages when checking the publishing conditions of events that have already met publishing conditions. We could reduce many of these log messages by not checking publishing conditions once RAVEN_ALERT has been applied to an external event.

This could be done by adding a return statement like

ext_labels = gracedb.get_labels(ext_id)
if 'RAVEN_ALERT' in ext_labels:
    return

around here: https://git.ligo.org/emfollow/gwcelery/-/blob/032d5ad061b4b6259d1e1933bf8c62487dea3a1a/gwcelery/tasks/raven.py#L386