Validate new Phurl URL

Summary: Closes T9691, Validate URL on Phurl objects for using valid protocols.

Test Plan: Create or edit URL. Change URL to "asdf" and observe error. Change back to "http://google.com" and observe no error.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Maniphest Tasks: T9691

Differential Revision: https://secure.phabricator.com/D14389