Consolidate file pasting code

Summary: Fixes T2861. This used to work but I think I broke it when I made the notification popup thing work. Merge it into the drag-and-drop since 90% of the code is the same anyway.

Test Plan: Pasted files into Chrome and got uploads. This doesn't work in Safari and Firefox; as far as I know it isn't supported in those browers.

Reviewers: blc, btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2861

Differential Revision: https://secure.phabricator.com/D5530