Added migration to reset existing relative_position for issues

db/post_migrate/20170309171644_reset_relative_position_for_issue.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class ResetRelativePositionForIssue < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def up
+    execute <<-EOS
+      UPDATE issues SET relative_position = NULL
+        WHERE issues.relative_position IS NOT NULL;
+    EOS
+  end
+
+  def down
+  end
+end