Joining a skeleton now requires that
    user is superuser
 or user owns the skeleton (and perhaps not all of its nodes)
 or user owns all the nodes of the target skeleton
 or the skeleton has a single node.

This removes a permissions loophole, whereby adding a node to an existing
skeleton and then merging that whole skeleton via the one node that the
user owns was possible. Identified by Casey and Ingrid.
Two more pie charts for statistics widget:
 * edited nodes not owned by the creator of that node
 * reviewed nodes
Merge remote-tracking branch 'origin/master'