Allow voting while session is still running

[SebastianAigner]

We've had multiple people mention that they could only vote for a session when it was already over.
Since some sessions might finish a few minutes early, this makes it easy for people to get distracted, do something else, and forget to vote for talks altogether. I'd propose allowing people to start voting for a talk 10 minutes or so before its scheduled end to address this situation 🗳️