Modernize chatlog a bit

Summary:
Ref T4786. This doesn't fully fix the issue since there's no way to make channels public yet, but gets some of the infrastructure more up to date.

  - Allow public access to the list and log controllers.
  - Implement proper policy checks in the Events (this has no practical impact on the only controller that loads this stuff, it's just for general/future purposes).
  - Remove a old-style unused method for building page frames.

Test Plan: Viewed log list and log details as logged-in and logged out users.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4786

Differential Revision: https://secure.phabricator.com/D8746