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/models/email_address.rb

@@ -0,0 +1,15 @@
+class EmailAddress < ApplicationRecord
+  # https://www.regular-expressions.info/email.html
+  EMAIL_REGEX = /\A[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}\z/
+
+  belongs_to :user
+
+  validates :address, presence: true, confirmation: true, format: { with: EMAIL_REGEX }
+  validates :normalized_address, uniqueness: true
+  validates :user_id, uniqueness: true
+
+  def address=(value)
+    self.normalized_address = EmailNormalizer.normalize(value) || address
+    super
+  end
+end