update nightscout web monitor

Howdy,

Using the green button to merge this pull request should update your repo to include our recent updates.
If you are also logged into Azure, you should observe a new deployment a few moments after merging.
Several moments after that, when the deployment complete (may take a few minutes), your website should update automatically.

(This adds a feature we're calling retrospective-forecasting, allows you to see whether or not your believe the forecasting).