users who no longer have ability to post lose a post if it is Quick Edited by someone else

[markjaquith]

Old bug, new place: #4458

Recycling my description:

1. User A is an Author, and published a post ("Post A").
2. User A is downgraded to "Subscriber"
3. User B edits "Post A" using the quick editing feature
4. Result: User A is no longer listed as the author of "Post A"

This is due to the fact that authors who currently lack the ability to save posts are not listed in the author dropdown box. We need to list the current author in that dropdown, regardless of capabilities, and ensure they are kept as the author on the backend.

This particularly comes to light when importing from another CMS, in which case all authors are created as Subscribers. Any editing of their posts will cause them to lose authorship.

Calling this one major, because mis-attributed posts are a big deal for professional publishers of content.

[automattor]

(In [12046]) Add the author to the quick edit dropdown author list if they currently lack the ability to edit so as to avoid stealing their post away from them. fixes #10967