R1:87871b358873

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…