Enable auto-completion of user names and paths on /createreview

Reusing code from /showreview.  To support auto-completion of paths, the
/getautocompletedata operation had to be extended, since it only supported
listing (relevant) paths per review (given a review id.)  Since we haven't
created a review yet, we have no review id.  Give /getautocompletedata a
list of changeset ids instead, which we do have.