Legalpad - make "Cancel" button "Log Out" button for required signature documents

Summary: Fixes T7299. Also re-direct the user to the initial request uri if the signature was required.

Test Plan: made a signature required legalpad doc. visit the instance at a specific uri, signed the document, and ended up at that specific uri

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7299

Differential Revision: https://secure.phabricator.com/D11809