Fix formatting for at-mentioned CCed users
Fix formatting for at-mentioned CCed users

Summary:
The webhook Slack sends formats the username including the angle
brackets (`<@username>`), so we respect that formatting and only apply
it as needed to bare usernames.

Test Plan: - CC a user with an at-mention and ensure it formats properly

Reviewers: dhruv

Reviewed By: dhruv

Differential Revision: https://phabricator.khanacademy.org/D36488

Khan/slack-deploy-hooks,Khan/slack-deploy-hooks