add index on content_migrations.context_id

Change-Id: I6a2f65a3f316806aa69adf0c54411e1ab35d25ca
Reviewed-on: https://gerrit.instructure.com/23523
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>

db/migrate/20130820210303_add_content_migration_index.rb

@@ -0,0 +1,12 @@
+class AddContentMigrationIndex < ActiveRecord::Migration
+  tag :postdeploy
+  self.transactional = false
+
+  def self.up
+    add_index :content_migrations, :context_id, concurrently: true
+  end
+
+  def self.down
+    remove_index :content_migrations, :context_id
+  end
+end