New TurnKey Ghost version (16.0)

Updated Ghost to latest upstream version - 3.15.0.
Update NodeJS to v12.x. Ghost recommends v12.x LTS NodeJS version; see: https://ghost.org/faq/node-versions/
Add ghost_user for maintainance more inline with upstream recommendations.
Explcitly disable TLS<1.2 (i.e. SSLv3, TLSv1, TLSv1.1). (v15.x TurnKey releases supported TLS 1.2, but could fallback as low as TLSv1).
Update SSL/TLS cyphers to provide "Intermediate" browser/client support (suitable for "General-purpose servers with a variety of clients, recommended for almost all systems"). As provided by Mozilla via https://ssl-config.mozilla.org/.
Updated version of mysqltuner script - now installed as per upstream recommendation.
Note: Please refer to turnkey-core's changelog for changes common to all appliances. Here we only describe changes specific to this appliance.