Email notifications for collective members

[rcsheets]

It would be helpful if members could opt in to email notifications of various types.

The first and probably most important example of this would be a notification in the event that a scheduled payment fails.

[rcsheets]

This was originally suggested in #53 but is being tracked separately here.

[PatrikD]

Instead of opt-in, I would make this the default behavior, or at most opt-out.

For extra credit, you could have a page of options, with a separate default setting for each.