update nightscout

Howdy,

This update includes several improvements to the code.
Caching to make Azure billing more predictable, a bg delta that gets displayed on the pebble watch, and a "retrospective forecast" to help evaluate whether or not to believe the blue dots.

To use this, follow the link to this "pull request", use the green Merge button to confirm the updates.
A few moments afterwards, Azure will create a new deployment, and a few minutes after that the website should update, all automatically.