Implementation of Schulze's Supermajority Requirements

Changes in detail:
- Renamed column "agreed" of table "initiative" to "majority"
- Added column "winner" to table "initiative"
- Extended constraint "non_admitted_initiatives_cant_contain_voting_results" of table "initiative" to "rank" and "winner" columns
- Removed constraint "non_agreed_initiatives_cant_get_a_rank" from table "initiative"
- Replaced PRIMARY KEY of "battle" table by a single NOT NULL constraint on column "issue_id" and three (partial) indicies allowing NULL values as initiative ids
- Added constraint "initiative_ids_not_equal" to "battle" table
- Added view "battle_participant" which selects admitted initiatives plus one virtual "status-quo" initiative for each issue denoted by an initiative_id being NULL
- Modified "battle_view" to include all admitted initiatives plus the virtual "status-quo" initiative
- Modified function "calculate_ranks" to respect all battle participants, to set the rank of all initiatives where the rank is better than status-quo, and to mark the final winner (if existent)