Allow for multi-select answers

[Matthew Jensen]

Allow for a poll that allows users to select multiple answers.

The most common use case for this is for polling the room for best available times for an event. When should we x? Pick all available dates.

[Andrew Macheret]

+1

As an example, facebook messenger polls allow multiple answers. Single-answer polls aren't even possible.

[Andrew Macheret]

Also very related, should be able to unvote. Currently this is not possible, but it would be trivial with multi-select answers.