notification_reason_text helper should not split the sentences to avoid breaking internationalization

notification_reason_text https://gitlab.com/gitlab-org/gitlab/blob/master/app/helpers/emails_helper.rb#L116 itself splits sentences which is a bad idea as mentioned https://docs.gitlab.com/ee/development/i18n/externalization.html#splitting-sentences

This is a follow-up of !17344 (comment 233140696)