Refactor seo data provision to script data

[thijsoo]

We want to introduce a way to gather all SEO data related to the current post/page/tax the person is editing and we want to provide that in a consistent way to the front end.

This issue is part of the bigger refactor of the way we supply our system with script data.

The solution that will be implemented will focus on first finding all the fields that are filled, looking for a way to gather those in a single object and then supplying that object to the front-end in a BC way so that it will work with the current front-end implementation.