Allow public countdowns to be viewed by logged out users

Summary: Fixes T5093. Ref T4830.

Test Plan:
  - As a logged out user, viewed a public countdown detail page.
  - (Tried to view a nonpublic one, got asked to login.)

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4830, T5093

Differential Revision: https://secure.phabricator.com/D9162