Quicksand and global drag and drop - install quicksand listener on event only

Summary: noticed this while doing some other work. didn't really break anything before but no need to install this listener every time we load the global drag and drop behavior; just the once works fine.

Test Plan: home -> profile -> back to home and drag and drop still worked

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12740