Check that Phurl alias is unique

Summary: Ref T8992, Phurl aliases must be unique. Otherwise throw an error.

Test Plan: Create two Phurl's both with alias 'asdf'. When saving second Phurl, form should show an error about the duplicate alias.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin

Maniphest Tasks: T8992

Differential Revision: https://secure.phabricator.com/D14401