Digest notifications should wait at least their interval before sending the first notification

[raamdev]

Forking this from https://github.com/websharks/comment-mail/issues/256.

Right now we have this behavior (quoting @jaswsinc):

The way it works is that a daily subscriber is sent email notifications no less than 24 hours apart. So it's OK for them to get an email as soon as a comment is posted, so long as they have not already been notified about any for the past 24 hours. If you then post another comment, it will be held and then delivered together with any others approx 24 hours later.

But that doesn't really make sense from a UX perspective. If a subscriber chooses "daily" notification, the expectation is that it will be at least a day before they get a notification (including their first notification).

We should wait at least 24 hours before the first notification for Daily Digests.

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Edit: The same goes with the Hourly and Weekly digest notifications. The first notification the subscriber receives should not go out until at least 1 hour or 1 week after they create the subscription.

If I subscribed and selected "weekly" as my digest option, and then 2 hours later someone comments and I receive a notification, I'm going to assume that the digest subscription is broken and not obeying my "weekly" selection.

[raamdev]

Noting that https://github.com/websharks/comment-mail-kb/issues/24 should be updated once this behavior changes.

[raamdev]

Related: https://github.com/websharks/comment-mail/issues/173