Add role=dialog to all dialogs

Summary: Seen some complaints about usability here, adding role=dialog to improve when these trigger.

Test Plan: Turn on Voiceover, tab over to log out link, here proper dialog title and text before highlighted submit button.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D15993