Add commit_id to draft_notes table

CE port of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14520

db/migrate/20190703043358_add_commit_id_to_draft_notes.rb

+# frozen_string_literal: true
+
+class AddCommitIdToDraftNotes < ActiveRecord::Migration[5.1]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def change
+    add_column :draft_notes, :commit_id, :binary
+  end
+end

db/schema.rb

@@ -1136,6 +1136,7 @@ ActiveRecord::Schema.define(version: 2019_07_31_084415) do
     t.text "position"
     t.text "original_position"
     t.text "change_position"
+    t.binary "commit_id"
     t.index ["author_id"], name: "index_draft_notes_on_author_id"
     t.index ["discussion_id"], name: "index_draft_notes_on_discussion_id"
     t.index ["merge_request_id"], name: "index_draft_notes_on_merge_request_id"

spec/db/schema_spec.rb

@@ -27,7 +27,7 @@ describe 'Database schema' do
     cluster_providers_gcp: %w[gcp_project_id operation_id],
     deploy_keys_projects: %w[deploy_key_id],
     deployments: %w[deployable_id environment_id user_id],
-    draft_notes: %w[discussion_id],
+    draft_notes: %w[discussion_id commit_id],
     emails: %w[user_id],
     events: %w[target_id],
     epics: %w[updated_by_id last_edited_by_id start_date_sourcing_milestone_id due_date_sourcing_milestone_id],