Email notifications ui

@vcmouli90 this PR hooks up the email setting UI. Can you do a quick review, merge it into your branch, and test it out?

Some of the changes I've had to do on the backend:

1. Added `_publish_attrs` and `VALID_TYPES` to `NotificationConfig `
2. `/api/set_active_notifications` endpoint that makes setting notifications on the front-end a lot easier.