Added 'disabled' attribute to checkbox field

Signed-off-by: Elliot Condon <e@elliotcondon.com>
added new 'Elements' option to relationship field to customise result HTML

Signed-off-by: Elliot Condon <e@elliotcondon.com>
updated CSS for new relationship featured image option

Signed-off-by: Elliot Condon <e@elliotcondon.com>
removed blank lines

Signed-off-by: Elliot Condon <e@elliotcondon.com>