user can group options and control a group's waps by a single slider #184
Description
grouping/ungrouping should be possible via the poll's or option's context menu and is purely participant-specific, allowing the participant to more effectively control waps of several options at once.
a group is sorted where the best-ranked member would appear. it can be expanded to show its members' sliders.
the group wap slider controls the median wap, shifting it preserves the members' differences in logit-transformed waps.
later, one might even form hierarchies of subgroups