I refactored getThreadOtherUsers: changed its name so it matches methods around and
make it return (Relative)MemberInfo's instead of IDs. It was only used in thread-utils.js and
it was easy to changed places where it was called. I need it in that form in typeahead, so I figured
it's fine to extend that function instead of creating another almost identical one.

Tested in furhter diffs.
Visible and works correctly returning all users excluding ashoat and viewer.
Performed sanity check if I didn't break the function (traced one of the functions (getSingleOtherUser) that called it and it performed as expected (in Relationship Prompt))