Quicksand - update help appropriately as you navigate about

Summary: Fixes T7724. Things are a bit tricky here as sometimes we don't want to display the help icon at all...! Change things such that we always render at least the icon, though it may have the style set to hidden if appropriate. Instrument quicksand javascript to then load the proper help dropdown if it can. Do this generally so other aphlict dropdowns could work pretty easily.

Test Plan: started on home and noted no help. clicked maniphest and saw maniphest help. clicked home and saw no help again. clicked differential and saw differential help.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7724

Differential Revision: https://secure.phabricator.com/D12499