Make owner autocomplete use the RT::Users->SimpleSearch API

Refactoring which was mistakenly omitted from d7163ec
Remove an unnecessary access of UserSearchFields

This also removes a misleading "default" value (which would never occur,
because of RT_Config's default) which also specified operators of '1'
instead of 'STARTSWITH'.
Allow searching and autocompleting users by custom fields
If there is but one user search result, redirect to it
Allow non-braced versions of CF.Name -- the braces are unnecessary here
Merge branch '4.2/user-autocomplete-cfs'