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/mailers/user_mailer.rb

@@ -4,11 +4,11 @@ class UserMailer < ApplicationMailer
 
   def dmail_notice(dmail)
     @dmail = dmail
-    mail(:to => "#{dmail.to.name} <#{dmail.to.email}>", :subject => "#{Danbooru.config.app_name} - Message received from #{dmail.from.name}")
+    mail to: dmail.to.email_with_name, subject: "#{Danbooru.config.app_name} - Message received from #{dmail.from.name}"
   end
 
   def password_reset(user)
     @user = user
-    mail to: "#{@user.name} <#{@user.email}>", subject: "#{Danbooru.config.app_name} password reset request"
+    mail to: @user.email_with_name, subject: "#{Danbooru.config.app_name} password reset request"
   end
 end