Add USA-specific models to interactive visualizations

[huddlej]

Context

We recently added a workflow to generate USA-specific nowcasts and forecast with the MLR model as part of a broader goal of submitting nowcasts to the SARS-CoV-2 variant nowcast hub. As part of this work, I chose not to upload the main model JSONs for the USA models to S3 or enable visualization of these models on the SARS-CoV-2 forecasts website.

Description

@trvrb suggested that it would be useful to visualize these model estimates on the website, so we should modify the USA-specific workflow to upload the main model JSONs to S3 and surface those results on the website.

Proposed solution

TBD. Some potential required steps will be:

1. Add a block to the variant hub workflow like this section in the main workflow to define paths to upload to S3
2. Add USA-specific models to the viz App by referencing the JSON files on S3 and added a section to the App to display the results