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/base.rb

@@ -320,7 +320,7 @@ class ImportScripts::Base
       opts[:username] = UserNameSuggester.suggest(opts[:username].presence || opts[:name].presence || opts[:email])
     end
 
-    unless opts[:email][EmailValidator.email_regex]
+    if !EmailAddressValidator.valid_value?(opts[:email])
       opts[:email] = fake_email
       puts "Invalid email '#{original_email}' for '#{opts[:username]}'. Using '#{opts[:email]}'"
     end