Open "Help" menu links in a new window

Summary: Fixes T11243. Seems reasonable to open this stuff in a new window so you don't put any application state in Herald, etc., at risk -- looking in this menu for help with a currently-executing workflow is reasonable and normal.

Test Plan: Clicked a help menu link, saw it open in a new page.

Reviewers: avivey, chad

Reviewed By: chad

Maniphest Tasks: T11243

Differential Revision: https://secure.phabricator.com/D16230