Allow applications to have multiple "help" menu items
Summary:
Ref T7199. Convert the single help menu item into a dropdown and allow applications to list multiple items there.
When an application has mail command objects, link them in the menu.
Test Plan:
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T7199
Differential Revision: https://secure.phabricator.com/D12244