Make review system support multiple users per treenode/connector

This feature was discussed in issue #580 and should be ready to merge now. There is a new `review` table that connects reviews to treenodes. The `reviewer_id` and `review_time` columns have been removed from the `location` table (and due to table inheritance from the `treenode` as well as the `connector` table). Of course, existing data is migrated. All widgets that refer to reviewing somehow have been changed to fit this.

These changes introduce a small API change: `compact-json` doesn't return the reviewer field anymore. So in case you have topic branches that rely on this, make sure they work with this or let me know if something is not clear.