By supplying a refreshurl for each event, retro.directory can automatically update the number of available places when a user clicks to see that event.
The URL will be queried server-side, meaning you can place any private information required in the query string and it will not be visible to the user.
The URL must return an HTTP 200 OK, and contain a JSON object looking like this:
retro.directory will rate-limit requests and query this a maximum once every 5 minutes, on demand.
Once the value of places reaches zero, further requests will no longer be made.
Note: this will not be used if showplaces=false
