Added check in poll card that allows setting the value of
results_enabled flag.
Access to stats/results now is controlled with abilities.
Polls related to proposals will be accessible to the proposal author
like they were administrators.
Back to polls has dissapeared. A link 'Back to proposal' will be shown
instead.
The button that invites the user to take part in other polls has been
removed.
Stats will flagged as disabled when a poll for a proposal is created.
