Add root_account_id to discussion_entry_participants

Closes PLAT-5736
flag=non

Test Plan
- Verify migrations run
- Verify you can set the root_account_id
  of a DiscussionEntryParticipant

Change-Id: Id9266ac7b38542d50dd9f39bd246eb2bf1131532
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/238593
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Rob Orton <rob@instructure.com>
QA-Review: Weston Dransfield <wdransfield@instructure.com>
Product-Review: Weston Dransfield <wdransfield@instructure.com>

db/migrate/20200527195732_add_root_account_id_to_discussion_entry_participants.rb

@@ -0,0 +1,32 @@
+#
+# Copyright (C) 2020 - present Instructure, Inc.
+#
+# This file is part of Canvas.
+#
+# Canvas is free software: you can redistribute it and/or modify
+# the terms of the GNU Affero General Public License as publishe
+# Software Foundation, version 3 of the License.
+#
+# Canvas is distributed in the hope that it will be useful, but 
+# WARRANTY; without even the implied warranty of MERCHANTABILITY
+# A PARTICULAR PURPOSE. See the GNU Affero General Public Licens
+# details.
+#
+# You should have received a copy of the GNU Affero General Publ
+# with this program. If not, see <http://www.gnu.org/licenses/>.
+
+class AddRootAccountIdToDiscussionEntryParticipants < ActiveRecord::Migration[5.2]
+  include MigrationHelpers::AddColumnAndFk
+
+  tag :predeploy
+  disable_ddl_transaction!
+
+  def up
+    add_column_and_fk :discussion_entry_participants, :root_account_id, :accounts
+    add_index :discussion_entry_participants, :root_account_id, algorithm: :concurrently
+  end
+
+  def down
+    remove_column :discussion_entry_participants, :root_account_id
+  end
+end