Added mailgunIsConfigured prop to config API
Added mailgunIsConfigured prop to config API

refs #10318

This will replace the isConfig property attached to the bulk_email_settings

JohnONolan/Ghost,janvt/Ghost,ErisDS/Ghost,ErisDS/Ghost,e10/Ghost,janvt/Ghost,TryGhost/Ghost,bitjson/Ghost,TryGhost/Ghost,ErisDS/Ghost,JohnONolan/Ghost,TryGhost/Ghost,e10/Ghost,JohnONolan/Ghost,bitjson/Ghost