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/logical/user_email_change.rb

@@ -8,11 +8,10 @@ class UserEmailChange
   end
 
   def process
-    if User.authenticate(user.name, password).nil?
-      user.errors[:base] << "Password was incorrect"
+    if User.authenticate(user.name, password)
+      user.update(email_address_attributes: { address: new_email })
     else
-      user.email = new_email
-      user.save
+      user.errors[:base] << "Password was incorrect"
     end
   end
 end