Ref T8989, Phurl "Visit URL" link should route through a separate controller.

Summary: Ref T8989, Phurl "Visit URL" should now route to an access controller that decides if the URL is valid whether to open it, or redirect back to Phurl object. New route is local.install.com/u/1 to open link.

Test Plan:

• open Phurl object with invalid URL, "Visit URL" link should redirect back to object
• open Phurl object with valid URL, "Visit URL" link should open the link
• open local.install.com/u/1 for U1 with valid URL should open the link
• open local.install.com/u/1 for U1 with invalid URL should redirect to local.install.com/U1

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: joshuaspence, Korvin

Maniphest Tasks: T8989

Differential Revision: https://secure.phabricator.com/D14381