Fix poll caching (fixes hhucn/adhocracy.hhu_theme#391)

Add session id to the cache key of poll and comment tiles.

This remedies CSRF alerts and thus broken poll widget, if a user
revisits cached tiles with a different session, e.g. by using a
different browser or logging out and in again.
Merge pull request #467 from nidico/feature-fix-poll-caching

Fix poll caching (fixes hhucn/adhocracy.hhu_theme#391)