DEV: pull email address validation out to a new EmailAddressValidator

We validate the *format* of email addresses in many places with a match against
a regex, often with very slightly different syntax.

Adding a separate EmailAddressValidator simplifies the code in a few spots and
feels cleaner.

Deprecated the old location in case someone is using it in a plugin.

No functionality change is in this commit.

Note: the regex used at the moment does not support using address literals, e.g.:
* localpart@[192.168.0.1]
* localpart@[2001:db8::1]

script/import_scripts/drupal.rb

@@ -76,7 +76,7 @@ class ImportScripts::Drupal < ImportScripts::Base
 
       create_users(users, total: user_count, offset: offset) do |user|
         email = user["email"].presence || fake_email
-        email = fake_email unless email[EmailValidator.email_regex]
+        email = fake_email if !EmailAddressValidator.valid_value?(email)
 
         username = @htmlentities.decode(user["username"]).strip