Show registration allowed in events api

Gijs Hendriksenrequested to merge
show-registration-allowed-in-events-api into master

Needed for ThaliApp#90.

Short description

Adds the registration_allowed field to the events API.

Also makes sure that the registration_allowed field in the Events model and the registered field in the EventListSerializer can no longer be None.

Steps to use this feature

  1. Visit /api/v1/events/.
  2. Notice each event has a boolean field registration_allowed that indicates whether registrations for this event are open.
  3. Notice that the field registered is true or false only, and no longer null.

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