Speed up the user admin page

The query was slow because it was had this clause:

ORDER BY `auth_user`.`username` ASC, `auth_customuser`.`user_ptr_id`

Having 2 tables in the ORDER BY clause makes things super slow, and
it's totally unneeded because username is a unique field.
Merge pull request #1859 from pculture/gh-1845

Speed up the user admin page