Render slowvote respondents as a facepile instead of smushed-together names

Summary: Make this more usable. Also fix a bug where $choices got overriden by a
loop variable.
Test Plan: Looked at a vote with multiple respondents.
Reviewed By: jungejason
Reviewers: tuomaspelkonen, jungejason, aran, codeblock
CC: aran, jungejason
Differential Revision: 629