Notification snooze feature.

**Purpose**
Snooze a notification to be reminded about it later.

**Use case**
- Similar to gmail’s snooze feature. This lets the user choose a better time to be notified about an important task.

**Requirements**
- A button on each row of the notification list.
- Clicking the button will show a dropdown with preset options
- Options: 1 hour, 1 day, 1 week
- A queue of future notification
- Cron to check the notification queue and send due notifications
- Implement https://github.com/DiscipleTools/disciple-tools-theme/issues/700 and https://github.com/DiscipleTools/disciple-tools-theme/issues/724 first

**Code considerations**
Start with `dt-assets/js/notifications.js`, `dt-notifications/notifications-endpoints.php`, `dt-notifications/notifications.php` and `template-notifications.php`.

Note: Making sure scheduled notifications are sent on time will be greatly improved by implementing a real cron service for each D.T instance: https://github.com/DiscipleTools/disciple-tools-theme/wiki/cron