allow users to select a once-daily digest of tracked items notifications

[davidmooreppf]

Just a thought, some casual users might prefer to receive one digest email of tracked notifications, as opposed to three or so emails per day from the API updates.

Obvs. would require developing ability to bundle notifications into one email, then was thinking it would be an option on /account/subscriptions/, to flip the switch for one daily digest, sent in morning.

Tag enhancement tag down the road tag no biggie