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).

script/fixes/063_migrate_emails.rb

@@ -0,0 +1,9 @@
+#!/usr/bin/env ruby
+
+require_relative "../../config/environment"
+
+User.where.not(email: nil).find_each.with_index do |user, n|
+  email = EmailAddress.new(user: user, address: user.email, is_verified: true)
+  email.save(validate: false)
+  puts "n=#{n} id=#{user.id} name=#{user.name} email=#{user.email} normalized_address=#{email.normalized_address}"
+end