[feeds_log] Make number of logged import entries to purge configurable

Interested in thought, feedback, anything :-)

Oh, wow. I hadn't imagined that more than 100 items would be imported on each cron run on average. I think it is a good idea to add a setting for this.

I do wonder if in feeds_log_cron() we should log how many items were removed from the log. That could potentially spam other logs. I see that dblog_cron() doesn't do this when cleaning up log entries.
At least I think that the \Drupal::messenger() bit could be removed. Or what is the reason that you added that?

Setting to "Needs work" because of the comment in #3:

At least I think that the \Drupal::messenger() bit could be removed. Or what is the reason that you added that?