Merge notification systems

[nlsvgtr]

The code contains two notification services.

The first can be found in ./src/services/notificationService.js which is used by ./src/services/eventService.js. This uses rulesets, and seems to be an attempt towards a generic and flexible notification system. The problem is that it is only used by Submissions.

The second can be found in ./src/notifications.js. It uses queues which are processed by crons. This one is used by the rest of the API.

These services should be merged.

[nlsvgtr]

On crete idea a mail is sent to the user. This is done in the route, using mail.sendThankYouMail. Newletter signups are followed by a sendNewsletterSignupConfirmationMail. These should be integrated in the notification system as well.