Simplify upvote/downvote implementation

Summary:
Use sigils to simplify the vote implementation and move most rendering to the server.

Use unicode glyphs in place of graphics.

Test Plan: {F19539}

Reviewers: pieter, starruler

Reviewed By: pieter

CC: aran

Maniphest Tasks: T1644

Differential Revision: https://secure.phabricator.com/D3518