Add option for field blocks and display mode - author & date fields

Description of the need We have added a number of great display suites type features. https://github.com/backdrop/backdrop-issues/issues/2084 https://github.com/backdrop/backdrop-issues/issues/1100

But, something that seems to be missing is the ability to add date and author information to display modes OR to access field blocks (through the layout system) for this information.

I was just experimenting with how much a site builder can modify their page with Backdrop CMS core and the inability to use existing features to move author and date information feels like an obvious omission.

Proposed solution

There would be options to arrange the author / date field on the display mode. There would be a field block for author and/or date submitted.

Alternatives that have been considered Describe alternatives you've considered. Have you seen this work in a better way in another CMS?

I have yet to look for contrib solutions.

Additional information Add any other information or screenshots that would help.

These are the field blocks currently available to users for the Post content type.