FIX: Conform EmailLog#bounce_error_code to RFC (#16010)

This commit makes sure that the email log's bounce_error_code
conforms to the SMTP error code RFC on save, so that
it is always in the format X.X.X or XXX without any
additional string details. Also included is a migration
to fix this issue for past records.

app/models/email_log.rb

@@ -14,6 +14,9 @@ class EmailLog < ActiveRecord::Base
     signup_after_approval
   }
 
+  # cf. https://www.iana.org/assignments/smtp-enhanced-status-codes/smtp-enhanced-status-codes.xhtml
+  SMTP_ERROR_CODE_REGEXP = Regexp.new(/\d\.\d\.\d|\d\d\d/).freeze
+
   belongs_to :user
   belongs_to :post
   belongs_to :smtp_group, class_name: 'Group'
@@ -34,6 +37,13 @@ class EmailLog < ActiveRecord::Base
     SQL
   end
 
+  before_save do
+    if self.bounce_error_code.present?
+      match = SMTP_ERROR_CODE_REGEXP.match(self.bounce_error_code)
+      self.bounce_error_code = match.present? ? match[0] : nil
+    end
+  end
+
   after_create do
     # Update last_emailed_at if the user_id is present and email was sent
     User.where(id: user_id).update_all("last_emailed_at = CURRENT_TIMESTAMP") if user_id.present?