Push Notifications: Interface for manual control over Notifications

[m-decarli]

It would be useful, to add a notification button to each event. For users this would trigger a notification, as soon as a free spot is available. Ideally, the notifications would be push notifications. But the choice of email notifications would be great as well.

For admins it would allow to better understand demand for events

[heddendorp]

Push notifications are covered in #92 Event bookmarks in #51

This issue should probably be changed to describe functionality that can be added after those issues are closed