Deprecate react-common's CovidMoratoriumBanner

[toolness]

Now that we're pulling the whole thing from contentful (see #32), we have no need of the component.

... but CovidMoratoriumBanner is currently the only component in our react-common package, so should we remove it, or hold on to it in case we eventually need common React components? Or do we actually want to use it to consolidate common Contentful rich text rendering logic (see #35)?