Fix Diffusion blame/highlight for logged-out users

Summary:
Fixes T5199. We try to save these options in user preferences, but logged-out users don't have preferences.

Instead, just use GET links for logged-out users.

Test Plan:
  - As a logged-out user, toggled blame and highlight on and off.
  - As a logged-in user, toggled blame and highlight on and off.

Reviewers: btrahan, vrana

Reviewed By: vrana

Subscribers: epriestley

Maniphest Tasks: T5199

Differential Revision: https://secure.phabricator.com/D9310