Fixed error in authentication.py: turns out that a list is larger than zero
instead of failing with an Exception.

Literally: in python, a list containing one tuple evals to larger than zero:

>> [('a',)][0] > 0
True

... which makes for on surface incomprehensible errors when the tuple contains a zero:

>> [(0,)][0] > 0
True

One of course has to use two sets of indexing brackets:

>> [(0,)][0][0] > 0
False
Make split_skeleton and _join_skeleton use can_edit_or_fail
which takes into account the user's groups.

While at it, simplified _join_skeleton a little.

Thanks to Matt Berck for pointing this out.
Merge branch 'master' of github.com:acardona/CATMAID