Add an alias to Phurl URL's

Summary: Ref T8992, Add an alias to Phurl URL's that can be used to redirect to link.

Test Plan: Add an alias to Phurl object, and navigate to local.install.com/u/<newalias>. This should redirect to the Phurl's URL.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: joshuaspence, Korvin

Maniphest Tasks: T8992

Differential Revision: https://secure.phabricator.com/D14395