R1:d8111f828f4d

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…