Improve insert behavior for drag-and-drop and paste uploads

Summary: In Safari, Firefox and Chrome, respect cursor position and selection ranges.

Test Plan: Dragged-and-dropped files into the middle of text, end of text, and a selected text range in Safari, Firefox and Chrome. Copy/pasted files into similar cases in Chrome. Got expected, normal behavior in all cases.

Reviewers: btrahan, vrana, jungejason

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1016

Differential Revision: https://secure.phabricator.com/D2155