Fix logo link in non-prod environments

Summary:
Using site.url will always link to the production environment,
which is both wrong and confusing when deployed in beta.
Interestingly, deploying with make serve appears to override
to localhost. The issue is only apparent in beta or when deployed
locally in a container.

Test Plan:
make run
Navigate to localhost:8080 and click the logo in the upper left. It should take you to localhost, instead of prod as it does pre-patch.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7412