3D viewer: fix wrong index for compact-json access

In commit ade90b8 I removed the reviewer field from the treenode representation
in the compact-json API. Therefore many indices had to change that are used to
access the response object. One index was forgotten, which is fixed by this
commit.
3D viewer: don't save reviews as node properties

Attaching the review information to each node hits performance significantly.
Therefore, this commit stores the returned review information as property of
each skeleton object. Shading methods can access this field, if they need it.
This also simplifies the delayed loading of the review information, which will
be added soon.

This is related to issue #668.