Badge / banner for participating sites

We should enable IEBS participating sites to show their users that they are part of the IEBS network. To do this, we can provide:
- A static banner / badge declaring their participation
- A dynamic banner or badge that lists which cities this site will receive updates for in the event of an emergency

Start by reviewing the repository's JavaScript entry points and how participating sites receive emergency updates. Confirm the expected static and dynamic badge surfaces with maintainers; done means sites can display participation and the relevant cities for emergency updates.

Written by the indexing model from the issue text.