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).

db/migrate/20200309043653_create_email_addresses.rb

@@ -0,0 +1,20 @@
+class CreateEmailAddresses < ActiveRecord::Migration[6.0]
+  def change
+    create_table :email_addresses do |t|
+      t.timestamps
+
+      t.references :user, index: false, null: false
+      t.string :address, null: false
+      t.string :normalized_address, null: false
+      t.boolean :is_verified, default: false, null: false
+      t.boolean :is_deliverable, default: true, null: false
+
+      t.index :address
+      t.index :normalized_address
+      t.index :user_id, unique: true
+
+      t.index :address, name: "index_email_addresses_on_address_trgm", using: :gin, opclass: :gin_trgm_ops
+      t.index :normalized_address, name: "index_email_addresses_on_normalized_address_trgm", using: :gin, opclass: :gin_trgm_ops
+    end
+  end
+end