USWDS Proposals - Make sure ADRs have a field that indicates what repo they apply to #54

Currently, this contains ADRs that intermingle the uswds/uswds repo and uswds/uswds-elements repo. If an ADR applies to a specific product, we should indicate that as consistent frontmatter in the ADR. Update all ADRs in https://github.com/uswds/uswds-proposals and the ADR template to contain this information.