Ensure all links end in `/`

[polymutex]

IPFS gateways require that links have a trailing slash in case they refer to their subdirectory's index.html.

Some gateways will detect this and do an HTTP redirect upon a 404 for which a {URL}/index.html exists, but this requires an additional roundtrip to the gateway and makes the page load slower.

$ curl -I https://beta.walletbeat.eth.link/wallet/summary 
HTTP/2 301 
[...]
location: /wallet/summary/

To avoid this, we should go over the codebase and ensure all links end in /, and we should add a test that verifies that this remains the case in the future.