additional poll type: combination of ingredients

[mensch72]

a group can pick an arbitrarily small or large combination of a fixed list of "ingredients". they basically state waps for ingredients, which determine approval for either the inclusion or exclusion of ingredients. if the slider meets the approval bar, inclusion is approved, otherwise exclusion is approved. an option can also be marked as "indifferent", in which case both inclusion and exclusion are approved.

given a user's approval for inclusion/exclusion of ingredients, we then say the user approves a certain combination of ingredients if and only if it approves the inclusion of all included and the exclusion of all excluded ingredients. after that, the normal maxparc tally procedure is applied to that, interpreting the combinations as options (so, using 2^k options if there are k ingredients). this gives the winning combination.

in addition to the possibility of tying ingredients similar to issue #183, users can also mark a number of ingredients and mark this set as "don't combine" via the context menu. if a user links some options A1,A2,...,Ak in this way, then she approves of a combination if and only if it excludes all ingredients she does not approve, and includes exactly one of those ingredients she does approve. this way a user can make it unlikely that options are combined that she thinks should not be combined.