Swap to iFrame for SARS-CoV-2 forecasts page

[trvrb]

Description of proposed changes

This commit switches from the currently PNG screenshots to a live iFrame embedding from https://nextstrain.github.io/forecasts-ncov/. This allows higher resolution and permits interactivity. I believe this is especially welcome for the lineage barchart where mousing over individual values is very helpful.

The iFrame width is responsive, but there is not perfect control over its height. That said, I believe my max(580px, 72%) solution works well at a variety of window sizes from vertical mobile to landscape desktop.

I also got rid of the disclaimer at the top of the page. This sort of MLR model is very standard at this point. It's used in the CDC Variant Tracker and we've spent a lot of time validating retrospective MLR frequency hindcasts, nowcasts and forecasts and I'm very happy with how well the model performs. I swapped this to a less aggressive warning directly above the iFrame plots.

Testing

Tested locally.

• [x] Checks pass

[trvrb]

Thanks for reviewing @joverlee521 🙂