Revisions: Wrong author shown for Jetpack/Atomic sites

Expected behavior

The expected behavior given how the API works now is that the app syncs all authors for a site when it syncs the site details. The revisions API includes the user ID for the author of each revision, and the app uses details from syncing the full list of authors to fill in the Gravatar and username for each revision.

Actual behavior

On Jetpack/Atomic sites, sometimes no author details or the wrong author details appear.

Case 1:

A user reported that on an Atomic site in the app, only one user (the admin/owner of the site) appeared next to all revisions on a post. However, an editor on the site had made some of those revisions, and in WP Admin the editor shows up correctly on those revisions.

In this case, the editor wasn't connected via Jetpack. This is a possible cause for all revisions being attributed to the admin user.

Case 2:

In various tests on Atomic and Jetpack sites, we have seen that no user info appears on the revision. It would be good to better understand why the author wasn't in the author list when the site was synced, and see if we can improve that experience.

cc @danielebogo
Internal ref: p4a5px-2m4-p2

Start by tracing revision author IDs through the site-details author sync on Jetpack and Atomic sites, using the two reported cases as reproduction targets. Compare revisions whose authors are not connected via Jetpack with revisions lacking author information, and consider the issue done when the correct author details appear consistently or the missing-author cause is documented with a testable fix.