parent/child theme detection

[mikemcguire]

Need detection to load templates from parent/child directories.

[lstanard]

@mikemcguire there's currently some logic in place to check for a child theme but it doesn't work correctly because THEME_CHILD_URI is set to the full URI via get_stylesheet_directory_uri(), which doesn't work with PHP's file_exists function. We can fix by using get_stylesheet_directory() instead, but it brings up another question.

This method checks if the /templates directory exists in the child theme, and if it does, uses that directory - so it's all or nothing. If a user wants to modify a single organism template they have to copy over the entire templates directory.

I think it would be better if we could look for each individual template in either the parent or child. I feel like that wouldn't be possible from within Nucleus, but would maybe need to happen within the parent theme somewhere. Thoughts?