DEV: Add SMTP group ID to EmailLog (#13381)

Adds a new `smtp_group_id` column to `EmailLog` which is filled in if the mail `from_address` matches a group's `email_username`. This is for easier debugging, so we know which emails have been sent via group SMTP.

db/migrate/20210614232334_add_smtp_group_id_to_email_log.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddSmtpGroupIdToEmailLog < ActiveRecord::Migration[6.1]
+  def change
+    add_column :email_logs, :smtp_group_id, :integer, null: true, index: true
+  end
+end