Date fields in outgoing webhooks now use the correct ISO8601 format

We've rolled out a fix for an issue where dates we presented in Webhooks were using an unusual ISO8601 format. This didn't match the one we document, or the one the REST API returned.

Previously, Webhooks returned dates like this: 2019-08-26 23:03:00 UTC 🙃

They're now consistent with our REST API, and will be returned in the format 2019-08-26T23:03:00.000Z 🎉