be more specific about fixing group_category_ids

it looks liek all of the "bad" data is 0, presumably from an
incorrect html form

also do the migration transactionally, to avoid the race condition
(the tables are small enough that a two-step FK addition isn't
necessary)

test plan:
 * run the migration; ensure it takes less than 1s, and doesn't
   fail

Change-Id: Ibe8ff590c41f3d07d51814564fefa8b986a78535
Reviewed-on: https://gerrit.instructure.com/12116
Reviewed-by: Brian Palmer <brianp@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>

db/migrate/20120626174816_constrain_assignment_group_category_ids.rb

@@ -1,14 +1,9 @@
 class ConstrainAssignmentGroupCategoryIds < ActiveRecord::Migration
   tag :postdeploy
-  self.transactional = false
 
   def self.up
-    Assignment.update_all({:group_category_id => nil}, <<-SQL)
-      NOT EXISTS (
-        SELECT group_categories.id FROM group_categories
-        WHERE group_categories.id=assignments.group_category_id)
-    SQL
-    add_foreign_key :assignments, :group_categories, :delay_validation => true
+    Assignment.update_all({:group_category_id => nil}, :group_category_id => 0)
+    add_foreign_key :assignments, :group_categories
   end
 
   def self.down