email for new inbox notifications

[cellio]

meta:277426 https://software.codidact.com/posts/290885

We should allow users to opt in to email notifications for new on-site (inbox) notifications. This is particularly helpful for "long tail" questions, where you get an answer or a comment months later, long after you've stopped actively monitoring it.

We already have machinery in place to send other kinds of notifications (daily or less often), so we should be able to build on that. We don't want to send email upon every new notification (too noisy), but maybe we could offer something finer-grained than daily?