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).

test/unit/spam_detector.rb

@@ -7,7 +7,7 @@ class SpamDetectorTest < ActiveSupport::TestCase
       SpamDetector.stubs(:enabled?).returns(true)
 
       @user = create(:gold_user, created_at: 1.month.ago)
-      @spammer = create(:user, created_at: 2.weeks.ago, email: "akismet-guaranteed-spam@example.com")
+      @spammer = create(:user, created_at: 2.weeks.ago, email_address: build(:email_address, address: "akismet-guaranteed-spam@example.com"))
     end
 
     context "for dmails" do