Break get_add_instance out of www_add; subclass it in Layout to
pull mobileTemplateId and mobileStyleTemplateId from the parent
if the parent is also a Layout.
Applied SquOnk's patch and added tests; modify www_add to inherit mobileStyleTemplateId
from the parent as well as mobileTemplateId. mobileTemplateId is only used by PageLayout
currently. If other assets start using this and this gets inherited cross asset types,
thing will break. A better design here would be to flag on a case-by-case basis in
asset definitions whether any given property should be inherited globally, of children
of the same asset class, or not at all.
