Add JS support for replacing Pholio mocks 1:1

Summary:
Ref T3572. Needs some CSS tweaks, but this lets you drag an image on top of another image to replace it. There's no server-side or transaction support (and I'm not planning to build that), I just wanted to clear the way on the JS side.

You'll get an additional array posted called `replaces`. Keys are old file PHIDs; values are new file PHIDs.

Note that a key may not exist yet (if a user adds an image, and then also replaces that same image). In this case, the server should just treat it as an add.

Test Plan: Dragged images on top of other images.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3572

Differential Revision: https://secure.phabricator.com/D6499