migrate labels to stars

test plan:

confirm that a previously labeled conversation is now starred, and that
they save without any errors

Change-Id: I36fab3f0406318fc2dd259c6db4f2da96cde9a48
Reviewed-on: https://gerrit.instructure.com/8330
Tested-by: Hudson <hudson@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>

db/migrate/20120126200026_labels_to_stars.rb

@@ -0,0 +1,9 @@
+class LabelsToStars < ActiveRecord::Migration
+  def self.up
+    ConversationParticipant.update_all("label = 'starred'", "label IS NOT NULL")
+  end
+
+  def self.down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end