By Sheila Dang, Katie Paul and Paresh Dave (Reuters) -Twitter Inc started a major round of layoffs on Friday, alerting employees of their job status by email after barring the entrances to offices and cutting off workers’ access to internal systems overnight. The move follows a week of chaos and uncertainty about the company’s future under new owner Elon Musk, the world’s richest person, who tweeted on Friday that the service was experiencing a “massive drop in revenue” as advertisers pulled spending. Musk blamed the losses on a coalition of civil rights groups that has been pressing Twitter’s…