DEV: Add created_via column to IncomingEmail (#11751)

This should make it easier to track down how the incoming email was created, which is one of four locations:

The POP3 poller (which picks up reply via email replies)
The admin email controller #handle_mail (which is where hosted mail is sent)
The IMAP sync tool
The group SMTP mailer, which sends emails when replying to IMAP topics, pre-emptively creating IncomingEmail records to avoid double syncing

db/migrate/20210119005647_add_created_via_to_incoming_email.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddCreatedViaToIncomingEmail < ActiveRecord::Migration[6.0]
+  def change
+    add_column :incoming_emails, :created_via, :integer, null: true
+  end
+end