Abstract submission: force exactly ONE author?

In one conference they want to have exactly one author, all the others should be co-authors. Is there a way to enforce this in the abstract submission? Sort of:

* default everybody to co-author (not author as it is now)
* check that there is 1 author (like it is already checked there is at least 1 speaker)

Or have a mandatory field for the author?

Start by locating the abstract submission handling for author and co-author roles, along with the existing validation that requires at least one speaker. Confirm how roles are defaulted and validated, then make the requested behavior complete when submissions enforce exactly one author and all other contributors are co-authors.

Written by the indexing model from the issue text.