Use MERGIN_BASE_URL to generate verify link in email

[nicogodet]

I use a nginx proxy with this config

    location / {
        proxy_pass         http://127.0.0.1:5000/;
        proxy_set_header   Host $host;
        proxy_set_header   X-Real-IP $remote_addr;
        proxy_set_header   X-Forwarded-Proto https;
        proxy_set_header   X-Forwarded-Host mergin.nicodet.fr;
        proxy_set_header   X-Forwarded-For $proxy_add_x_forwarded_for;
    }

In email, the verify link is http