Allows page wrappers to have one default setting per node type, rather than one global default setting

Per one client's request, we needed the ability to have page wrapper default settings per node type, so that, for instance, Donation Forms have one set of default page wrappers and Articles have another. This comes from a request in which the client wanted to enable 'Article' as a page-wrapper-enabled node type but was finding it frustrating to physically reset the page wrapper default settings on every article (since the vast majority of articles will not have page wrappers at all).

This primarily changes the page wrappers admin page so that the default settings are per enabled node type, rather than having one global setting. The "add another" button, which has an ajax callback, has been refactored to work properly with this new arrangement.

The variables stored by the page_wrappers module are per node setting, so variable names are, e.g. "page_wrapper_default_config_donation_form" rather than "page_wrapper_default_node_config".

I do want to highlight one issue I'm still having, which is that if I click "Add another" on the settings form for one node type, then click "add another" for a second node type, then save the form, it does not save the new setting for the first node type.