Better WP_User_Query search

[gluten]

The get_users() function only searches 'user_login' and 'user_nicename' (when searching for a user by name). There is no way to search users by their display name. Including 'display_name' would make searching with get_users() much more intuitive, since in my experience users tend to be known by their display name and not necessarily their login. In theory I could do a meta_query for fields such as first and last name, but I can't find an easy way to query 'display_name' since it is not on the 'usermeta' table.

Add display_name to WP_User_Query search columns

Makes sense.

This won't scale as there is no index on the column in the database, see #15170, #16620 and #16366.