Improve email address validation (#29838)

app/validators/email_address_validator.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+# NOTE: I initially wrote this as `EmailValidator` but it ended up clashing
+# with an indirect dependency of ours, `validate_email`, which, turns out,
+# has the same approach as we do, but with an extra check disallowing
+# single-label domains. Decided to not switch to `validate_email` because
+# we do want to allow at least `localhost`.
+
+class EmailAddressValidator < ActiveModel::EachValidator
+  def validate_each(record, attribute, value)
+    value = value.strip
+
+    address = Mail::Address.new(value)
+    record.errors.add(attribute, :invalid) if address.address != value
+  rescue Mail::Field::FieldError
+    record.errors.add(attribute, :invalid)
+  end
+end