OutputPage::headElement adds attributes to the body element via 2 methods - a hook and skin.
I'd like to settle on the hook here to make it more obvious where these attributes are coming from and deprecate the addToBodyAttributes method.
- Vector updated to use hook (with check against skin)
- Minerva updated to use hook (with check against skin)
- Remove the Skin::addToBodyAttributes method but call it with a deprecation notice if the method exists. (Note: nothing seems to use parent:: so this should be fine)