Summary: Fixes T9117. Adds a migration to remove ponder vote data.
Test Plan: I added a bunch of lines to phabricator_user.edge with type 18 and they were successfully removed by this patch
Reviewers: #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: epriestley, yelirekim
Maniphest Tasks: T9117
Differential Revision: https://secure.phabricator.com/D16452
