Show "Login to Answer" in Ponder if viewer is logged out

Summary: Fixes T9278. Logged out viewers shouldn't see a form field to answer, just a login button.

Test Plan: Log out, go to question, click Login to Answer, login, get redirected back.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T9278

Differential Revision: https://secure.phabricator.com/D14012