* Added option to form a relationship between a SrcSet group and a custom image size
* Added logic to render_img_tag_html() method to first check for a srcset group assigned to the current image size, then to fallback and check if the image size is in fact a srcset group, failing that just render the html as usual
* Removed redux option in general section to select a srcset group to use for posts/pages as this is now redundant due to the fact that custom image sizes can now be related to srcset groups