Create block for USWDS site alert

Update from weekly scrum: Steve is working on this, but it ended up being more involved than expected due to the way the base module structured the data. He is restructuring the data controller so it passes through more of the field values.

I have the block existing, and the new endpoint that has all the data we need.

I am going to bypass the AC of "Block displays only sitewide alerts of style USDS Site Alert" at this time because all of the filtering and placing of alerts happens in the javascript and I think it will be easier to handle modifying the javascript logic once we have all three blocks.

This block as it exists is not yet using the new data endpoint.

This is as complete as can be until we work on the theming and replicating the javascript.

I also updated the home page with the new version of the Readme.

Automatically closed - issue fixed for 2 weeks with no activity.