Extend featured image support to blog categories

Users who want to use the featured image background tool or the featured image widget tool for their layout cannot set a featured image for their blog categories.

Instead, each featured image tool will use the latest blog post image from that category, which is too dynamic for some purposes...

To eliminate the need to create individual templates for each blog category, we could:

allow a featured image in the WordPress "Posts > Categories" screen, or we could
leverage the featured image that users can supply for grids in the "ProPhoto > Settings > Grids > Category Featured Images" area.

downshiftorg / prophoto-issues