R1:f2c36a934ec8

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…