Write gateway links to a single docker compose file on gateway to simplify management

[thebalaa]

Currently, create_link.sh launches a standalone container on the gateway for each link, instead lets write links to the same docker compose so the gateway operator can easily modify links without having to recreate them, for example when changing the domain(s) a link serves.

This change will also make it possible to backup all of gateway's link for easy recreation when upgrading or migrating to a new host.