Spin-off from #1364784: Monthly Interval varies by a few days.

If editions have been skipped (for ex if cron didn't run for some time), then only the most recent due edition is sent.

Should we add an option to send them all?

Example:

- edition sent 1 Jan 2012
- cron fails to run through all of Feb due to server problems
- cron runs on 1 Mar 2012

Should this send only the edition for 1 March (as it currently does) or both 1 Feb and 1 March? This very much depends on what how the site uses newsletters, hence best left as an option.

(My take on it is that I'm not really sure there'd be circumstances where you'd want the catch-up editions sent, but filing this to discuss it.)