[UX][D8] Create/select layout for a specific page from the content add/edit form.

This is the "sibling" issue of #2894 and part of the #345 meta-issue. Also part of #378. See https://github.com/backdrop/backdrop-issues/issues/1131#issuecomment-435... for details/screenshots.

Original issue summary

Related to #1126

I would like to see another vertical tab that allows people with proper permissions to select a layout for a single piece of content right there from the content add/edit form. This should populate/be reflected to the respective layout visibility conditions.