Systemd enable/disable behaviour

[ErisDS]

In order to fully setup Ghost with systemd, we need to call systemctl enable <servicename>.

Short term, we're going to do this as part of setup, just to get the call in to the codebase.

Longer term, we need a slightly more elegant solution.

The problem with enabling during setup, is that if the user chooses not to start Ghost, it will be started if the server is then restarted. This is a bit odd.

Instead, we need some more elegant logic to handle calling enable on the first start, if enable has not already been called.

The suggested logic is:

• systemd could add something like an --enable option to ghost start and a --disable option to ghost stop
• systemd would check if it's enabled on ghost start
• if it's not and --enable is not passed, prompt the user and ask if they want to enable
• if it is enabled do nothing regardless of whether or not --enable is passed (edited)
• for stop, it wouldn't disable unless --disable is passed (edited)
• the stop --disable part is for people who are stopping a blog but not uninstalling it