users: move emails to separate table.

* Move emails from users table to email_addresses table.
* Validate that addresses are formatted correctly and are unique across
  users. Existing invalid emails are grandfathered in.
* Add is_verified flag (the address has been confirmed by the user).
* Add is_deliverable flag (an undeliverable address is an address that bounces).
* Normalize addresses to prevent registering multiple accounts with the
  same email address (using tricks like Gmail's plus addressing).

app/logical/spam_detector.rb

@@ -12,7 +12,7 @@ class SpamDetector
 
   attr_accessor :record, :user, :user_ip, :content, :comment_type
   rakismet_attrs author: proc { user.name },
-                 author_email: proc { user.email },
+                 author_email: proc { user.email_address&.address },
                  blog_lang: "en",
                  blog_charset: "UTF-8",
                  comment_type: :comment_type,