Support receiving notifications by email

[ttencate]

Not to be confused with #375 which is about global broadcasts. This issue is about pulling people back to the site after they've already joined an event and received some feedback on it.

We promised on the sign-up form that all email would be opt-in except in "exceptional circumstances". So maybe something like this:

Notify me by email of new notifications:

• Immediately
• At most once a day
• Never (default)

And maybe it's exceptional enough to warrant a one-off email to everyone, to tell them about this shiny new feature. That email in turn could be done through #375, which I think is the more important one of this pair.