Added personal voting

Added seat model.
Added motion poll detail view and link on motion detail view.
Added new slide for motion polls. Fixed apply command in KeypadUpdate view and KeypadCreate view.
Added config field for live voting.
Added projector message for live mode and non live mode.
Removed config variable for message when a poll is closed.
Updated gitignore.
Added Norman to Authors.
Updated templates for OpenSlides 1.6.x branch.
Small other changes.
Used serialnumber sended by VoteCollector 1.3.4.
Changed meta files.
Updated German translation.
Templates reworked for personal voting.

Updated README.
Fixed pep8.
Merge pull request #11 from emanuelschuetze/PersonalVoting

Personal voting
Cleared poll when votecollector is run again. Moved CSS for votecollector message zu overlay template file. Added PDF for personal vote results.
Improved vote result PDF for single votes.

- Allow that seat.number is null (for empty seats).
- Updated translations.
Merge pull request #12 from emanuelschuetze/PersonalVotingFixes

Personal voting fixes