Add Canonical tag configuration to the Yoast settings form.

Canonical URL is important for SEO and for user sometimes it is not clear how to set it and where. So it will be more convinience to manage it upon the Yoast settings form. Canonical URL settings is already in the Metatags settings, so we need just to add settings to the Yoast form and store it into the corresponding Metatag settings field(see Robots settings issue → ).