Remove all code referencing old tab navigation

Summary:

• Add getHelpURI() to PhabricatorApplication for application user guides.
• Add a new "help" icon menu item and skeletal Diviner application.
• Move help tabs to Applications where they exist, document the other ones that don't exist yet.
• Grep for all tab-related stuff and delete it.

Test Plan: Clicked "help" for some apps. Clicked around randomly in a bunch of other apps.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D3267