Fix poll.vote and not poll.show

Currently, the link to poll details goes to an empty overlay if users can vote, but cannot see the poll results.
Furthermore, the voting authorization depended on show, to which there is no reason.
This commit remedies both situations.
Show no up/down elemnets if user can neither see current state nor vote herself
Merge pull request #217 from phihag/fix-poll-rate-but-not-show-auth

Fix poll.vote and not poll.show
Merge branch 'develop' into milestone_easier_buildout_merge_develop