Conpherence - tweak global upload + durable conpherence interaction

Summary:
Ref T7539. This stuff is pretty whack - the "dragEnter" and "dragLeave" fire over and over and over despite not moving the mouse sometimes from JX.Mask interaction, causing that neat flickering effect. Fix this mess by disabling pointer events for the mask.

Test Plan: dragged a file to the durable column textarea. it uploaded and inlined the Fxxx into the message. tried uploading a few times and it worked repeatedly.

Reviewers: epriestley, chad

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7539

Differential Revision: https://secure.phabricator.com/D12058