Remove reviewer retrieval from fetch_treenodes view

This view isn't used anywhere in the upstream code base right now.
Adapting it to the new multi user review system is therefore pointless
at the moment. If this functionality is needed in this view, it can be
added later without anymore problems.
Make fetch_treenodes view support multi user review

Instead of returning a list which optionally contains the reviewer_id in each
entry, this method now optionally adds a list of reviewers to each entry.

This commit is part of the changes discussed in issue #580.