Fix error in permissions to join two skeletons.
Fix error in permissions to join two skeletons.

An 'elif' was an 'if', disabling skeletons present under fragments
or under the user's staging area from being merged.

fzadow/CATMAID,htem/CATMAID,fzadow/CATMAID,htem/CATMAID,fzadow/CATMAID,htem/CATMAID,htem/CATMAID,fzadow/CATMAID