Change: send events to webhook as JSON with a schema.

[arikfr]

Example of resulting event:

{
   "data":{
      "user_id":null,
      "created_at":"2017-02-02T08:08:15+02:00",
      "object_type":"test",
      "org_id":1,
      "object_id":null,
      "additional_properties":{
         "test":1
      },
      "action":"test"
   },
   "schema":"iglu:io.redash.webhooks/event/jsonschema/1-0-0"
}

Closes #1552.

[arikfr]

@alexanderdean there are two changes here from your schema:

• No id field. I prefer to not include it, in case in the future we might not store these events to the local database. If you still want an ID for deduplication purposes, we can add UUID maybe?
• additional_properties is an object and not a string field.

[alexanderdean]

Sounds good @arikfr - I've added snowplow/iglu-central#484

Snowplow can't (yet) deduplicate webhook events based on webhook-specific fields, so no rush to add in an event UUID from our side (I agree however that it's better to expose a UUID than a current local database sequence)...

[alexanderdean]

Not a deal-breaker but I would probably emit the created_at using UTC - so in this case it would be:

"created_at":"2017-02-02T06:08:15Z",

It's slightly more compact, and less brittle - the TZ of the instance running Redash shouldn't be significant at analytics time...

[arikfr]

In most deployments the server time will be UTC anyway. I'll change it to be UTC regardless, but I guess it's less urgent.

[alexanderdean]

Makes sense - thanks @arikfr !