Multisite cron-runner

[Clorith]

Currently, we rely on WP-CLI to trigger any cron events where needed, but this means keeping tabs on all subsites if a multisite is set up, and spawning a cron event to run WP-CLI on that subsite as well.

Instead, we should leverage the fact taht all cron tasks run on the server, and create a tool that's bundled, and deployed, with Project Base. This should loop over and trigger the WP-CLI crons on each subsite in a network where appropriate.

If this is implemented, to account for latency, heavy loads etc, cron tasks should only trigger every 5 minutes, instead of every minute. This is a DevOps part of it all, but just making a note for the future.

Alternatively, we may want to look at solutions like Cavalcade for larger networks.