As a user, I want to set my availability while I'm attending a event

[renaudboyer]

When creating/editing an event, I want to set if I'm available or not while attending the event.

Description

When other users want to invite me to an event, my availability is displayed in the attendee list and they are notified if they want to invite me if I'm already attending another event.

When I'm attending an event, by default I'm unavailable.

As a user, I want to set if I'm available or not when I create an event.

UI

Add a select next to Visibility with values:

• Busy as default
• Available

Behavior

When Busy EVENT ical TRANSP value should be set to OPAQUE
When Free EVENT ical TRANSP value should be set to TRANSPARENT

Notes

• Availability is leveraging the TRANSP PROPERTY of the event (RFC5545 here)
• For now, we'll add this feature on Event I've organized as we can't modify event user is attending.