Non exclusive UNIQUE market - custom positions

ONLY one outcome. This will be the new default for binary events.

if outcome[0].position_labels == "custom":

    render vertical list of the two positions. First row is the long, second  row is short. The labels should respect outcome[0].long_label and outcome[0].short_label.

else: # yes/no, up/down

    render outcomes horizontally, where the first one is the long and second one is short.

When user bet, the payload should always include the position, and the outcome will be always outcome[0].id.