Provide an <input type="file"> control in Remarkup for mobile and users with esoteric windowing systems

Summary:
Ref T5187. This definitely feels a bit flimsy and I'm going to hold it until I cut the release since it changes a couple of things about Workflow in general, but it seems to work OK and most of it is fine.

The intent is described in T5187#176236.

In practice, most of that works like I describe, then the phui-file-upload behavior gets some weird glue to figure out if the input is part of the form. Not the most elegant system, but I think it'll hold until we come up with many reasons to write a lot more Javascript.

Test Plan:
Used both drag-and-drop and the upload dialog to upload files in Safari, Firefox and Chrome.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T5187

Differential Revision: https://secure.phabricator.com/D15953