R1:c169199e6429

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:
{F355925}

{F355926}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7199

Differential Revision: https://secure.phabricator.com/D12244