Fixed #20088 -- Changed get_admin_log not to depend on User id field

Before this change, the get_admin_log method would expect User model's
FK to be named `id`. When changing that FK name, admin/index.html
rendering would fail.

This includes:
 * Changed the use of id for the use of pk property.
 * Added a regression test that fails without the patch.

This commit refs #20088.
Merge pull request #931 from catalanojuan/fix-admin-log-dependency-on-user-id-field-20088

Fixed #20088 -- Changed get_admin_log not to depend on User id field