Sort election in a more advanced way

As described in issue #8 (closed) the elections are now sorted in a more advanced way in both the management as well as the vote frontend. There are four categories in the following order:

• Open elections: Elections in which the voters are currently allowed to vote
• Upcoming elections: Elections that have not yet started.
• Published results: Closed elections for which the admin has published the results.
• Closed elections: Closed elections for which the results have not (yet) been published.