Internet Explorer compatibility fixes

Summary:
A few fixes for IE that makes this work in IE9+ and at least you can drag things and have it fail horribly in IE8 now. Ref T5023.

• try / catch around getBoundingClientRect because IE throws if the object is not attached to the DOM. The correct value can be inferred in these cases as 0,0
• turns out IE has different values for button here (1,2,4) than other browsers. special case this by using the other IE fact that 'which' will be null

Test Plan: dragged and dropped tasks successfully in IE9, IE10, and IE11 on Windows 7. dragged and dropped tasks unsuccessfully in IE8 on Windows 7 - they always "drop" to the 2nd position or so as I think the ondrag code isn't firing properly or something

Reviewers: chad, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5023

Differential Revision: https://secure.phabricator.com/D9118