Send outgoing notification immediately when their UpdateInterval elapses

[mjl-]

When we find a new module, and it matches a subscription, we'll send an email to the user. Except when we already sent a message within the past user-configured interval. In that case, we store that we have a module to notify about. It will be included in the next attempt to send a notification. So when a next new module is found. That may take a while.

We should be notifying as soon as the interval elapses. Probably by putting a "send at" timestamp in the ModuleUpdate, and periodically querying the database for those that are due.