Add Monthly fixed (system-wide) event

True, a sysop could add/configure a monthly timed event instead, but that
requires more configuration. Fixed events are nice in their simplicity.

This is intended for running the new Synchronet trashman utility, but could be
used for anything that's simple to execute and the sysop:
- doesn't care which node/instance of sbbs it runs from
- doesn't care exactly what time the event runs
- doesn't need the assorted options available for timed events