Show a "Logout?" dialog when the user goes to /logout/

Summary: Currently, the logout link is this awkward form in the footer since we have to CSRF it. Enable a GET + dialog + confirm workflow instead so the logout link can just be a link instead of this weird mess.

Test Plan: Went to /logout/, logged out.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D3066