API - "Measurement system" replaced with "sensor units"

✨ Improvements

As we are moving away from using measurement system (Metric or Imperial) to define sensor units in Webhooks,
and making it possible to select units per sensor instead, we're making some updates to the API

In summary:

• measurementSystem is removed from the response to requests about webhooks
• sensorUnits is mandatory when adding a webhook

Changes to the webhook system is documented in the Webhook changelog.