Fix localization of polls

[lukasrad02]

The polls feature needs better integration into page localizations:

• [ ] Allow localizing options and option descriptions
• [ ] Don't treat translated polls as separate polls
  • [ ] A user who voted on a German poll should not be able to vote on the corresponding English poll and vice versa
  • [ ] Results for a poll should take votes from the German and English variant into account
• [ ] Synchronize properties like the eligible groups (see #373 and the fix in #491)