Quicksand - make things work correctly with global drag and drop upload

Summary: Fixes T7685. This required making the global drag and drop behavior able to "uninstall" itself so to speak, and then it re-installs it self as necessary.

Test Plan:
Did the following all successfully

• uploaded a file to homepage
• homepage -> differential -- no way to upload via drag and drop
• homepage -> differential -> homepage -- uploaded a file
• homepage -> differential -> browser back button to homepage -- uploaded a file

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7685

Differential Revision: https://secure.phabricator.com/D12534