Add modified poll plugin

[kjlaw]

• Adds voting groups (workers and requesters) for single choice polls
• Retains ability to not group voters for single and multiple choice polls
• Type of poll can be chosen in the poll creator modal (checkbox sets ability to group voters)
• Requires admin to fill in two settings: "poll voter info endpoint" (https://daemo.org/v1/user/activity/) to get info about whether the user is an active worker or requester, and "poll constitution link" which directs users who are not eligible to vote to the Daemo Constitution for more information
• Note that this is not currently compatible with the original poll plugin on Discourse