R1:e40aa8f78249

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…