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

lib/email/receiver.rb

@@ -152,7 +152,8 @@ module Email
         imap_uid_validity: @opts[:imap_uid_validity],
         imap_uid: @opts[:imap_uid],
         imap_group_id: @opts[:imap_group_id],
-        imap_sync: false
+        imap_sync: false,
+        created_via: @opts[:source].present? ? IncomingEmail.created_via_types[@opts[:source]] : nil
       )
     end