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
