Review system: make review update support multi users

This commit updates the review update backend function to write into the new
review table and therefore to support multiple users per treenode.

This is part of the work on issue #580.
Review system: update node info to support multiple reviews

This commit updates the front and backend for providing treenode information
(e.g. by pressing <I> when a treedenode is selected). It now supports multiple
reviewers per node and displays them as a list.

This is part of the work on issue #580.