add context index to external tools

Change-Id: I91c635c0f33c43835c39fef1ff9495b2040abb95
Reviewed-on: https://gerrit.instructure.com/87912
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>

db/migrate/20160816165844_add_context_index_to_external_tools.rb

@@ -0,0 +1,8 @@
+class AddContextIndexToExternalTools < ActiveRecord::Migration
+  disable_ddl_transaction!
+  tag :postdeploy
+
+  def change
+    add_index :context_external_tools, [:context_id, :context_type], algorithm: :concurrently
+  end
+end