Fix skeleton graph calculation by using new review system

If the risk calculation was selected, the skeleton graph calculation was
still expecting the old review system. This commit fixes this problem
and is part if issue #580. Thanks to Stephan Gerhard for pointing this
out.
Graph widget: fix node coloring for risk calculation

The back-end provides a single skeleton ID for each node, but the fron-end was
expecting a list. This commit makes the front-end use the single skeleton ID
provided by the back-end instead of the first list element.