Refactor and test user event serialisation

[pcapriotti]

This replaces the ad-hoc serialisation code for user event with schema-profunctor instances, like everything else, and introduces golden tests for them. The JSON golden files were generated using the old code to ensure this refactoring hasn't changed anything.

https://wearezeta.atlassian.net/browse/WPB-6892

Checklist

• [x] Add a new entry in an appropriate subdirectory of changelog.d
• [x] Read and follow the PR guidelines