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

spec/components/imap/sync_spec.rb

@@ -78,6 +78,8 @@ describe Imap::Sync do
         .and change { Post.where(post_type: Post.types[:regular]).count }.by(1)
         .and change { IncomingEmail.count }.by(1)
 
+      expect(IncomingEmail.last.created_via).to eq(IncomingEmail.created_via_types[:imap])
+
       expect(group.imap_uid_validity).to eq(1)
       expect(group.imap_last_uid).to eq(100)