Scheduling of live events

[smirolo]

A LiveEvent model can be attached to PageElement to indicates that the element in a sequence is a webinar instead of an online reading step.
The user will have to register for the Webinar at LiveEvent.location. Once the user will have attended the webinar, the webinar instructor will mark the EnumeratedProgress for the user as passed (i.e. setting viewing_duration to the duration of the webinar).

Side note: We will want to have repeating meetings as Calendars (ex: .ics) in the future so some integration with a CalDAV server will be required (ex: Radicale).

[smirolo]

The following features are still missing from pull request #84:

• API endpoints to attach/create or cancel new live event

Bug to fix: The API endpoint to mark an user attendance picks the closest (in time) LiveEvent. The live event should be specified as part of the API call, because there are many use cases were an instructor might not be able to record attendance before 2 classes have been run - whether they were given back-to-back, or simultaneously by different instructors.