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/factories/user.rb

@@ -4,7 +4,6 @@ FactoryBot.define do
       "user#{n}"
     end
     password {"password"}
-    email {FFaker::Internet.email}
     default_image_size {"large"}
     level {20}
     created_at {Time.now}