T6870, Return to previous page after login via header

Summary: Fixes T6870, logging in from a public object should land on that object.

Test Plan: Navigate to a maniphest task in a logged out state, login, landing page should be maniphest task.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T6870

Differential Revision: https://secure.phabricator.com/D11289