Pass on signals to pubsweet server

[erezmus]

When starting pubsweet server, the cli script in pubsweet/cli/server.js spawns a new node process using pubsweet/src/startup/start.js as entry point.

We need the cli script to listen to SIGTERM and on that signal terminate the child process

Pubsweet issue: https://gitlab.coko.foundation/pubsweet/pubsweet/issues/443