Allow event registering for open events

/label feature priority: low

One-sentence description

Allow event registering for open events

Motivation

So that the personal calendar can contain these events, notifications only get send to these people and open events have an indication how many people are going to go to the event.

Desired functionality

The normal registration functionality. But it is not registration, more like Facebooks

Suggested implementation

We should be able to add an option that if there is no registration required that you can still register.