Fix query frequency unit and change time preference from input to dropdown.

Summary: Ref T8362, Fix query frequency unit and change time preference from input to dropdown.

Test Plan: Change user time preference in Date Time Settings panel, open feed, observe new time stamps.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8362

Differential Revision: https://secure.phabricator.com/D13236