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/user_deletion.rb

@@ -29,7 +29,7 @@ class UserDeletion
   end
 
   def clear_user_settings
-    user.email = nil
+    user.email_address = nil
     user.last_logged_in_at = nil
     user.last_forum_read_at = nil
     user.favorite_tags = ''