Fix several issues with "Logout"

Summary:
Ref T1536.

• When we render a dialog on a page by itself, put it on a dust background.
• Currently, we render "Logout" in two different places. Stop doing that.
• Make sure the surviving one has workflow so we get a modal ajax dialog if possible.

Test Plan: {F46731}

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6226