Create ballots for voting events via API

[dpausp]

Other components of Ekklesia, namely ekklesia-portal, want to set up ballots for voting through a JSON API.
It must be possible to create multiple ballots with one API call. The API has to generate a UUID for each created ballot so other components can refer to it.

• Input: list of ballot configs
• Output: list of ballot UUIDs generated by ekklesia-voting if all configs are valid, else error information
• Voting system, start time, end time and user roles which are allowed to vote must be configurable.
  • Must support voting systems: STV, ranked candidate list and simple yes/no questions.
• Ballots must support competing options (all competing options = one ballot).
• Voting events can be represented as a separate entity.
• Ballot configuration must be immutable.