IPFS retrieval: endpoint update

[salgozino]

In a code review, I've found that we have hardcoded some urls to ipfs.klers.io. Specifically:

• list
• policy
• registry

This hardcoded values should use the envvar IPFS_GATEWAY defined here to retrieve those values.
Please, update them in order to be consistant in the site and to have control over the gateway that we are using.
Thanks!