Do not return unpublished destinations IDs on related events from API

[flibbertigibbet]

For JSON destination results from the search API endpoint (and any other endpoints that may return events as JSON), check if the related destination is published, and omit its ID from the response if not.

This is to address an issue where the Android app crashes because the related destination cannot be found (only published destinations are returned).

Also, it would be good to ensure the drop-down for associating an event with a destination only displays published destinations as options, although that will be insufficient to protect against the destination getting unpublished later.