List active ballots for voter

[dpausp]

As a voter, I want to see on the index page all ballots which I can currently vote on and for how long this will be possible.

The voter has to have one of the roles required by the ballot and the ballot must be active.
Ballots are active when the current time is between start and end time of a ballot and the ballot has not been cancelled.

--
The voter sees a ballot in this list, if:

• the ballot is active
  • current time between start and end time of ballot
    • adapt ballots query
  • not cancelled
    • create new table column "canceled_at" (type DateTime) and adapt test db
• the voter has one of the roles required by the ballot
  • class EkklesiaAuthData.roles -- see ekklesia-portal

[sunkup]

I think we should not show the ballots on the index page, but rather some general / relevant information and the login button. So we show the index page to users which are not logged in / have logged out. After logging in, the user can be redirected to "/ballots" automatically, but the "index" and "ballots" pages should be available in the menu.

[dpausp]

Sounds good, let's do that :)