The dashboard vhost, the hive domain vhost and the dashboard's local
name move to hive-c0re/vhosts.nix; the matrix .well-known discovery and
the legacy /matrix/ redirect move to glue-matrix-hive-discovery.nix,
which adds them to the hive domain vhost. Both declare through
gateway.vhosts. hive-gateway/vhosts.nix keeps the catch-all only, and
the gateway no longer reads deploy.matrix or c0re.servedFrontend, nor
deploy.hive-controller.enable for its vhosts.
Part of #3742.
