Make default content author field autocomplete the username

Problem/Motivation

While reviewing #3123393: Users are duplicated (Integrity constraint violation) Duplicate entry → , I noticed the Username of default content author expects a username value, but it has to be entered manually. It would be nice to have an autocomplete widget for selecting users similar to the one presented in node forms to select the node author.

Steps to reproduce

Go to admin/structure/migrate/wordpress_migrate
Upload a WXR/XML file
Go to the next step and select No for the Create new users for existing WordPress content authors? question.
You shall see the Username of default content author field.

Proposed resolution

Change the form element to an autocomplete user widget.

Data model changes

The autocomplete will save the user id as an integer instead of a string with the username. Changes in the wizard might be necessary.