Require opt-in for tracking cron activity

[fjarrett]

There are several system-wide events that run on cron jobs, such as cleaning out trashed posts once per week and deleting spammed comments that are older than 30 days.

Depending on the site size, and the number of spam comments it receives, these cron jobs can potentially create a lot of records that aren't pertaining to changes being made to live content, but rather are pertaining to old or unnecessary content being routinely purged.

This means that records for important changes being made to live content can be drowned out by the extra noise created by cleanup events.

We should not remove cron-authored activity by any means, but this should be an opt-in only feature for those who specifically require it.

Also see this forum post for a related discussion where another plugin was performing a lot of cron events and it was causing Stream to become bloated.