Refactor dangerous query in cron job

[sauloperez]

The definition of the send_recent_posts_to_online_members cron job fetches all organizations present in the DB in #23 (diff), which will surely blow up our database sooner or later.

We must do this differently and at least process them in batches.

[markets]

@sauloperez fixed in coopdevs/timeoverflow#401

[mperezv]

Fixed here.

[markets]

Thanks @mperezv! yep that was fixed in the linked PR, but not yet tested -> merged -> deployed.