Add options to hide and disable livepreview window

[jkevingutierrez]

There is a Toggle live preview button that shows and hides the livepreview window, but when you load a page it always shows the window by default. It would be great to have an option in settings HIDE_LIVEPREVIEW to hide the window by default when someone is editing a page.

Also, sometimes you want to show livepreview in some environments (Staging and Dev) and hide it in others (Production). It would be very useful to have another option in settings DISABLE_LIVEPREVIEW to disable livepreview globally.