Don't show a "Manage" button in Legalpad if the user is signing a TOS document

Summary:
When a TOS-like Legalpad document is marked "Require this document to use Phabricator", the login prompt shows a "Manage" button, but that button doesn't work.

When we're presenting a document as a session gate, don't show "Manage".

Test Plan: Viewed a required document during a session gate (no "Manage" button) and normally (saw "Manage" button).

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20312