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]

lib/validators/email_address_validator.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class EmailAddressValidator
+  def self.valid_value?(email)
+    email.match? email_regex
+  end
+
+  def self.email_regex
+    /\A[a-zA-Z0-9!#\$%&'*+\/=?\^_`{|}~\-]+(?:\.[a-zA-Z0-9!#\$%&'\*+\/=?\^_`{|}~\-]+)*@(?:[a-zA-Z0-9](?:[a-zA-Z0-9\-]*[a-zA-Z0-9])?\.)+[a-zA-Z0-9](?:[a-zA-Z0-9\-]*[a-zA-Z0-9])?$\z/
+  end
+end