Currently the easiest way to make sure the post meta
is added to the Gutenberg editor, since the featured
image metabox hook/filter is no longer available.
Presumably this will eventually be replaced with a true
block, or whatever it will be, but this works for the moment.
Initial work to change `_displayfeaturedimagegenesis_disable` to a select with image size options instead of being a checkbox. Options will be:
* 0: default (consistent with checkbox)
* 1: disable (consistent with checkbox)
* 2: backstretch (new)
* 3: large (new)
Robin Cornett