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