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/email.rb

@@ -10,7 +10,7 @@ module Email
 
   def self.is_valid?(email)
     return false unless String === email
-    !!(EmailValidator.email_regex =~ email)
+    EmailAddressValidator.valid_value?(email)
   end
 
   def self.downcase(email)