Add snowplow events for timelines (#21008)

* Add a json schema

* Pass required data for snowplow events

* Pass required data for snowplow events

* Add snowplow event tracking for Timeline event creation

This changes the timeline_event API endpoint to send snowplow tracking when an event is created.
The FE now sends additional information for this purpose:

- source: collections or question. This is null if the event is added via API
- question_id: added if the source is from a question. null if the question is unsaved.

These are dissoc-ed from the body before the db/insert because the backend does not store this information, it's just
from the FE and only relevant for this tracking data.

* Fix json error

* Add ::new-event-created entry to the snowplow test

* Fix json error

* Fix json error

* Fix snowplow test

Co-authored-by: Adam James <adam.vermeer2@gmail.com>
Co-authored-by: adam-james <21064735+adam-james-v@users.noreply.github.com>