Remove specific route used for favorites, as there is no need to display a new page; rework add/remove link management; add comments
Let the theme decides of the class to include into the link, just add the action performed for a styling purpose
Remove getFavoriteButton template tag and let the theme displays its own buttons, just add a new isFavorite template tag for the theme to know what to show (add or remove)
Simplify favorites management from theme point of view: only post id should be passed through the add/remove link and ThemeApp functions