Add config option to limit user registrations to certain email domains

This feature was requested long ago (see http://feedback.gitlab.com/forums/176466-general/suggestions/4118466-ability-to-register-only-from-ceratain-domains) and I recently needed to implement it as well. Hope to see it in future release.

This MR adds a config option to config/gitlab.yaml.example, initializes this option to default value in 1_settings.rb initializer, adds a validation to user.rb model based on this config option, adds tests to test out this validation.