Set user as default sort order for user auto-complete lists

Created by: allistera

Currently when Gitlab gets a list of users (For example in the Assignee field for a merge request) for a autocomplete select field it orders them by date created, it would be must more elegant if this was sorted by the users name.

It looks like this was previously the case, but the code has been changed since then: https://github.com/gitlabhq/gitlabhq/pull/3645