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

app/models/incoming_email.rb

@@ -29,6 +29,15 @@ class IncomingEmail < ActiveRecord::Base
     SQL
   end
 
+  def self.created_via_types
+    @types ||= Enum.new(
+      handle_mail: 1,
+      pop3_poll: 2,
+      imap: 3,
+      group_smtp: 4
+    )
+  end
+
   def to_addresses_split
     self.to_addresses&.split(";") || []
   end
@@ -83,6 +92,7 @@ end
 #  imap_uid          :integer
 #  imap_sync         :boolean
 #  imap_group_id     :bigint
+#  created_via       :integer
 #
 # Indexes
 #