Allow a domain other than the install domain to serve as a short Phurl domain

Summary: Ref T8995, config option for Phurl short domain to share shortened URL's

Test Plan:

• Configure Phurl short domain to something like "zz.us"
• Navigate to zz.us; get 404
• Navigate to zz.us/u/3 or zz.us/u/alias where U3 is an existing Phurl; redirect to correct destination

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin

Maniphest Tasks: T8995

Differential Revision: https://secure.phabricator.com/D14447